Make HWUI makefile include external/skia/src/utils

Test: none. No new code; this just makes it explicit that HWUI is
depending on headers in Skia's src/utils directory.

This is needed for SkTextureCompressor.h, which itself is pulled in
transitively through SkGlyphCache.h. Adding this here means that Skia
need not list src/utils as an exported include dir.

Change-Id: I3f2b022504486b7a640236fb98a497b599b42a6b