build: Support SDL_sound in different prefix as SDL2

As the required SDL_sound is a custom fork which is not packaged as
system package, it could plausibly be installed in a different path.

CMakeLists.txt

@@ -427,6 +427,7 @@ target_include_directories(${PROJECT_NAME} PRIVATE
 	${PIXMAN_INCLUDE_DIRS}
 	${PHYSFS_INCLUDE_DIRS}
 	${SDL2_INCLUDE_DIRS} # Blindly assume other SDL bits are in same directory
+	${SDL_SOUND_INCLUDE_DIRS}
 	${Boost_INCLUDE_DIR}
 	${MRI_INCLUDE_DIRS}
 	${VORBISFILE_INCLUDE_DIRS}