Remove unnecessary dependency, and fix the chain

Test: No new tests. This should not affect behavior; only dependencies.

Font.h does not need to include SkGlyphCache.h, and doing so requires
a transitive dependency on external/skia/src/utils, which was not
intended. Forward declare it instead, and fix the build errors that
resulted.

Change-Id: Ifd09430bb848d51b139df0f0c06c63e7e48711eb
diff --git a/libs/hwui/pipeline/skia/SkiaPipeline.cpp b/libs/hwui/pipeline/skia/SkiaPipeline.cpp
index 6606b02..5ba0f42 100644
--- a/libs/hwui/pipeline/skia/SkiaPipeline.cpp
+++ b/libs/hwui/pipeline/skia/SkiaPipeline.cpp
@@ -18,6 +18,7 @@
 
 #include "utils/TraceUtils.h"
 #include <SkImageEncoder.h>
+#include <SkImagePriv.h>
 #include <SkOSFile.h>
 #include <SkOverdrawCanvas.h>
 #include <SkOverdrawColorFilter.h>