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
diff --git a/include/osmocom/mgcp_client/Makefile.am b/include/osmocom/mgcp_client/Makefile.am
index 0eaafcc..d303ddf 100644
--- a/include/osmocom/mgcp_client/Makefile.am
+++ b/include/osmocom/mgcp_client/Makefile.am
@@ -9,3 +9,5 @@
 mgcp_common.h: $(top_srcdir)/include/osmocom/mgcp/mgcp_common.h
 	echo -e "/*\n\n   DO NOT EDIT THIS FILE!\n   THIS IS OVERWRITTEN DURING BUILD\n   This is an automatic copy of <osmocom/mgcp/mgcp_common.h>\n\n */" > mgcp_common.h
 	cat $(top_srcdir)/include/osmocom/mgcp/mgcp_common.h >> mgcp_common.h
+
+CLEANFILES = mgcp_common.h