SDL: Fix some warnings in sw-sdl.c
SDL: Fix some warnings in sw-sdl.c

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