build: make sure copied mgcp_common.h is removed on 'make clean'

I thought the BUILT_SOURCES also enters the files to CLEANFILES, but 'make
clean' leaves the copied header behind for me. Add to CLEANFILES to get it
removed.

Change-Id: I5a56f83289e32a09643047f0e779c9de3e4b39a6
1 file changed