sdl2_mixer: build fix.
sdl2_mixer: build fix.

miqlas/haikuports,miqlas/haikuports,nielx/haikuports,haikuports/haikuports,moochris/haikuports,moochris/haikuports,nielx/haikuports,Begasus/haikuports,koletzky/haikuports,haikuports/haikuports,Begasus/haikuports,Begasus/haikuports,koletzky/haikuports,haikuports/haikuports,nielx/haikuports,moochris/haikuports,Begasus/haikuports,haikuports/haikuports,miqlas/haikuports,koletzky/haikuports,Begasus/haikuports,haikuports/haikuports