hal: fix hal compile error

Add AM_CFLAGS AUDIO_GKI_ENABLED.

Change-Id: I50a5202cac12979b44a46c2d1632a546ed70a88f
diff --git a/post_proc/Makefile.am b/post_proc/Makefile.am
index 8bd41ae..993058e 100644
--- a/post_proc/Makefile.am
+++ b/post_proc/Makefile.am
@@ -14,6 +14,8 @@
 AM_CFLAGS += -DAFE_PROXY_ENABLED
 endif
 
+AM_CFLAGS += -DAUDIO_GKI_ENABLED
+
 if HW_ACC_EFFECT
 AM_CFLAGS += -DHW_ACCELERATED_EFFECTS
 c_sources += hw_accelerator.c