"make dist" fix for: no rule to make mgcp_common.h

Mark osmocom/mgcp_client/mgcp_common.h as nodist, so "make dist" will
not try to include it in the source tarball. This caused "make dist" to
fail in a clean osmo-mgw source tree with:
make[2]: *** No rule to make target 'osmocom/mgcp_client/mgcp_common.h', needed by 'distdir'.  Stop.

The file gets copied during make from osmocom/mgcp/mgcp_common.h (see
include/osmocom/mgcp_client/Makefile.am). Therefore it is not included
in the source tree and we don't need to distribute it.

Related: OS#4084
Change-Id: Ia1d7b051c0924a785b0f7ec0195192e3a852ed70
diff --git a/include/Makefile.am b/include/Makefile.am
index 2daaf20..fbbd08d 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -6,9 +6,14 @@
 	osmocom/mgcp_client/mgcp_client.h \
 	osmocom/mgcp_client/mgcp_client_endpoint_fsm.h \
 	osmocom/mgcp_client/mgcp_client_fsm.h \
-	osmocom/mgcp_client/mgcp_common.h \
 	osmocom/mgcp/mgcp.h \
 	osmocom/mgcp/mgcp_common.h \
 	osmocom/mgcp/mgcp_internal.h \
 	osmocom/mgcp/osmux.h \
 	$(NULL)
+
+# This gets copied during make from osmocom/mgcp/mgcp_common.h. Therefore it is not included in the source tree and we
+# don't need to distribute it (OS#4084).
+nobase_nodist_include_HEADERS = \
+	osmocom/mgcp_client/mgcp_common.h \
+	$(NULL)