Fix vpx_rtcd.h dependency in Android.mk

Failed to build on Linux (as described in Android.mk) with NDK r7b.
Set vpx_rtcd.h dependency after libvpx sources are added to
LOCAL_SRC_FILES so that vpx_rtcd.h is generated before any libvpx file
is touched.

Change-Id: Ibe19d485ca9f679dc084044df0e3fb14587c4d3e