drm_hwcomposer: Fix build warnings

A cleanup patch to fix the build warnings that have surfaced
from the new build system.

BUG=None
TEST=Builds without warnings

Change-Id: I2ad898e627d451f87705d2bdf06a315b8e1c7a1c
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/drmdisplaycomposition.h b/drmdisplaycomposition.h
index 3c94e6b..3ab934d 100644
--- a/drmdisplaycomposition.h
+++ b/drmdisplaycomposition.h
@@ -32,7 +32,7 @@
 
 namespace android {
 
-struct SquashState;
+class SquashState;
 
 enum DrmCompositionType {
   DRM_COMPOSITION_TYPE_EMPTY,