Edit source code to remove compilation warnings, to allow -Werror (easy)

[mmadia]

This is a generalized ticket to expose an area to contribute.

Several portions of Haiku's code is not compiled with -Werror. Those portions can be found in ​build/jam/BuildSetup.

Some of them only fail on GCC 2 vs. both GCC's. Take care to test with both compilers. As of hrev45297 these are those directories:

#	EnableWerror src add-ons accelerants intel_extreme ;
#	EnableWerror src add-ons accelerants matrox ;
#	EnableWerror src add-ons accelerants nvidia ;
#	EnableWerror src add-ons accelerants radeon ;
#	EnableWerror src add-ons accelerants radeon_hd ;
#	EnableWerror src add-ons input_server filters ;
#	EnableWerror src add-ons input_server methods ;
#	EnableWerror src add-ons kernel bus_managers acpi ;
#	EnableWerror src add-ons kernel bus_managers firewire ;
#	EnableWerror src add-ons kernel bus_managers ps2 ; # gcc2
#	EnableWerror src add-ons kernel debugger ; # gcc2
#	EnableWerror src add-ons kernel drivers audio ;
#	EnableWerror src add-ons kernel drivers graphics ;
#	EnableWerror src add-ons kernel drivers input ;
#	EnableWerror src add-ons kernel drivers network ;
#	EnableWerror src add-ons kernel drivers power ;
#	EnableWerror src add-ons kernel file_systems ext2 ;
#	EnableWerror src add-ons kernel file_systems fat ;
#	EnableWerror src add-ons kernel file_systems googlefs ;
#	EnableWerror src add-ons kernel file_systems ntfs ;
#	EnableWerror src add-ons kernel file_systems reiserfs ;
#	EnableWerror src add-ons kernel network datalink_protocols ;
#	EnableWerror src add-ons kernel network stack ;
#	EnableWerror src add-ons media media-add-ons usb_webcam ;
#	EnableWerror src add-ons media media-add-ons video_producer_demo ;
#	EnableWerror src add-ons media plugins asf_reader ;
#	EnableWerror src add-ons media plugins avi_reader ;
#	EnableWerror src add-ons media plugins ffmpeg ;
#	EnableWerror src add-ons media plugins matroska ;
#	EnableWerror src add-ons media plugins mov_reader ;
#	EnableWerror src add-ons media plugins mp3_reader ;
#	EnableWerror src add-ons media plugins ogg ;
#	EnableWerror src add-ons media plugins raw_decoder ;
#	EnableWerror src add-ons media plugins speex ;
#	EnableWerror src add-ons media plugins wav_reader ;
#	EnableWerror src add-ons translators exr ;
#	EnableWerror src add-ons translators hpgs ;
#	EnableWerror src add-ons translators jpeg ; # gcc2
#	EnableWerror src add-ons translators png ; # gcc2
#	EnableWerror src add-ons translators stxt ;
#	EnableWerror src add-ons translators wonderbrush ;

Mention this ticket in your Gerrit change requests, preferably in the "Blocking:" field.

[pulkomandy]

I think this makes a good easy task and could be added to the next run of Google Code-In.