libwebp modules should depend on their makefile

This is to make sure that any change to the makefile triggers rebuilding
of the module.

(cherry picked from commit d68e528902de4473052751486098fc10737338cf)

Change-Id: I1cde729bba94c3d507f308f5131bb50f260e1277
Merge "libwebp modules should depend on their makefile"
Merge branch 'master' of https://android.googlesource.com/platform/external/webp into cm-11.0