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/drmdisplaycomposition.h b/drmdisplaycomposition.h
index 3ab934d..264963c 100644
--- a/drmdisplaycomposition.h
+++ b/drmdisplaycomposition.h
@@ -21,7 +21,7 @@
 #include "drmhwcomposer.h"
 #include "drmplane.h"
 #include "glworker.h"
-#include "importer.h"
+#include "platform.h"
 
 #include <sstream>
 #include <vector>