Fix warning from clang (class -> struct).

Change-Id: I298a062b161177f34849a4526a817d0b978efe60
diff --git a/driver/rsdFrameBufferObj.h b/driver/rsdFrameBufferObj.h
index c6e7deb..e388e2d 100644
--- a/driver/rsdFrameBufferObj.h
+++ b/driver/rsdFrameBufferObj.h
@@ -19,7 +19,7 @@
 
 #include <rsContext.h>
 
-class DrvAllocation;
+struct DrvAllocation;
 
 class RsdFrameBufferObj {
 public: