SDL_mixer : Specify branch SDL-1.2 for download

kivy#19

Default SDL_mixer branch now wants to build version 2, which causes both the SDL_mixer build to fail, and consequently, the kivy build as well.

Xcode Frameworks will show libSDL_mixer.a and libkivy.a are missing.

Specifying the SDL-1.2 branch results in the build working, fixing both problems.

tools/build-sdlmixer.sh

@@ -9,7 +9,7 @@ if [ ! -f SDL ]; then
 fi
 
 if [ ! -d SDL_mixer ]; then
-	try hg clone http://hg.libsdl.org/SDL_mixer/
+	try hg clone http://hg.libsdl.org/SDL_mixer/#SDL-1.2
 fi
 if [ ! -d libtremor ]; then
 	try mkdir libtremor