Fix format string mismatch. Clang build fix.

The gralloc usage flags are an enum, which is backed by an
implementation defined type. Both gcc and clang are making this a long
long. All the values are hard coded to fit within 8 hex characters
though, so just make the cast.

Change-Id: I4a9778a24c2cbf7fb663371d208e30c4921e170d