Add compile define for specific backends only in CMake

Fixes https://github.com/mackron/miniaudio/pull/1010#issuecomment-3093830252Signed-off-by: Marcin Serwin <marcin@serwin.dev>

CMakeLists.txt

@@ -156,6 +156,8 @@ if(MINIAUDIO_NO_NULL)
     list(APPEND COMPILE_DEFINES MA_NO_NULL)
 endif()
 if(MINIAUDIO_ENABLE_ONLY_SPECIFIC_BACKENDS)
+    list(APPEND COMPILE_DEFINES MA_ENABLE_ONLY_SPECIFIC_BACKENDS)
+
     if(MINIAUDIO_ENABLE_WASAPI)
         list(APPEND COMPILE_DEFINES MA_ENABLE_WASAPI)
     endif()