drm_hwcomposer: Move importer.h to platform.h

We're going to need more platform-specific stuff, so lump it
all in the same header.

BUG=b/28117135
TEST=compiles

Change-Id: Idfe82e9a29a0bcd284a02b2e58af0e620fdc542b
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/drmcompositor.h b/drmcompositor.h
index fd057c0..27623d9 100644
--- a/drmcompositor.h
+++ b/drmcompositor.h
@@ -19,7 +19,7 @@
 
 #include "drmcomposition.h"
 #include "drmdisplaycompositor.h"
-#include "importer.h"
+#include "platform.h"
 
 #include <map>
 #include <memory>