drm_hwcomposer: fix build error after fbf5c0ca45b3

'warning' preprocessor directive is interpreted as build error.
Replace it with ALOGW.

Fixes: fbf5c0ca45b3 ("drm_hwcomposer: libdrm gralloc_handle: modifiers and YUV support")
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/platform/platformdrmgeneric.cpp b/platform/platformdrmgeneric.cpp
index ed3a68c..95a1eac 100644
--- a/platform/platformdrmgeneric.cpp
+++ b/platform/platformdrmgeneric.cpp
@@ -238,7 +238,11 @@
   bo->hal_format = gr_handle->format;
 
 #if GRALLOC_HANDLE_VERSION < 4
-#warning libdrm >= v2.4.97 is required to support modifiers
+  static std::once_flag once;
+  std::call_once(once, []() {
+    ALOGE(
+        "libdrm < v2.4.97 has broken gralloc_handle structure. Please update.");
+  });
 #endif
 #if GRALLOC_HANDLE_VERSION == 4
   bo->modifiers[0] = gr_handle->modifier;