Fix "hardware/qcom/audio/post_proc/effect_api.c:67:17: error: use of undeclared identifier 'EINVAL'"

Change-Id: I09c96c1a0b6f57c42969a375fedb0cb5b0b66c40
diff --git a/post_proc/effect_api.c b/post_proc/effect_api.c
index cf3968b..9c15e8f 100644
--- a/post_proc/effect_api.c
+++ b/post_proc/effect_api.c
@@ -17,6 +17,7 @@
 #define LOG_TAG "offload_effect_api"
 //#define LOG_NDEBUG 0
 
+#include <errno.h>
 #include <stdbool.h>
 #include <cutils/log.h>
 #include <tinyalsa/asoundlib.h>