drm_hwcomposer: Propagate PLATFORM_SDK_VERSION to cflags

PLATFORM_SDK_VERSION is needed for already present
code (GetDisplayIdentificationData/GetDisplayCapabilities),
which implements some of the android.hardware.graphics.composer@2.3 API.

Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: I1a5019e9d8ae327db41f55cee0ba3636f913928f
diff --git a/Android.bp b/Android.bp
index 85be0b5..8bcd1aa 100644
--- a/Android.bp
+++ b/Android.bp
@@ -61,6 +61,12 @@
         "-DHWC2_INCLUDE_STRINGIFICATION",
     ],
 
+    product_variables: {
+        platform_sdk_version: {
+            cflags: ["-DPLATFORM_SDK_VERSION=%d"],
+        },
+    },
+
     relative_install_path: "hw",
     vendor: true,
 }