GBA: Fix GCC warnings
GBA: Fix GCC warnings

cassos/mgba,sergiobenrocha2/mgba,askotx/mgba,jeremyherbert/mgba,Touched/mgba,iracigt/mgba,AdmiralCurtiss/mgba,iracigt/mgba,jeremyherbert/mgba,Touched/mgba,Iniquitatis/mgba,bentley/mgba,mgba-emu/mgba,mgba-emu/mgba,AdmiralCurtiss/mgba,matthewbauer/mgba,libretro/mgba,libretro/mgba,jeremyherbert/mgba,Iniquitatis/mgba,sergiobenrocha2/mgba,bentley/mgba,libretro/mgba,fr500/mgba,sergiobenrocha2/mgba,matthewbauer/mgba,Anty-Lemon/mgba,fr500/mgba,mgba-emu/mgba,zerofalcon/mgba,libretro/mgba,Anty-Lemon/mgba,cassos/mgba,askotx/mgba,sergiobenrocha2/mgba,iracigt/mgba,zerofalcon/mgba,Anty-Lemon/mgba,zerofalcon/mgba,askotx/mgba,MerryMage/mgba,jeremyherbert/mgba,Iniquitatis/mgba,askotx/mgba,cassos/mgba,Touched/mgba,mgba-emu/mgba,iracigt/mgba,MerryMage/mgba,Iniquitatis/mgba,MerryMage/mgba,sergiobenrocha2/mgba,libretro/mgba,fr500/mgba,Anty-Lemon/mgba,AdmiralCurtiss/mgba,fr500/mgba