omni: replace cflags by export_cflags

Needed for GRALLOC_HANDLE_HAS_RESERVED_SIZE to be exported correctly

Change-Id: Iaea6e3a4caa0730403054cef9ff5a0b6453ea7f2
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index 3c5a0b8..8e93edd 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -197,14 +197,14 @@
     module_type: "cc_defaults",
     config_namespace: "omniGlobalVars",
     bool_variables: ["gralloc_handle_has_reserved_size"],
-    properties: ["cflags"],
+    properties: ["export_cflags"],
 }
 
 gralloc_handle_reserved_size {
     name: "gralloc_handle_has_reserved_size_defaults",
     soong_config_variables: {
         gralloc_handle_has_reserved_size: {
-            cflags: ["-DGRALLOC_HANDLE_HAS_RESERVED_SIZE"],
+            export_cflags: ["-DGRALLOC_HANDLE_HAS_RESERVED_SIZE"],
         },
     },
 }