build: move mgcp/*.h to noinst_HEADERS, drop RPM libosmo-mgcp-devel

We only install the mgcp_client/ headers.

Related: OS#6300
Change-Id: Ie0f79222bd1702097c12193dcf7a0462805cfc4a
diff --git a/include/Makefile.am b/include/Makefile.am
index 0b66cb3..c72cba8 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -8,6 +8,9 @@
 	osmocom/mgcp_client/mgcp_client_endpoint_fsm.h \
 	osmocom/mgcp_client/mgcp_client_fsm.h \
 	osmocom/mgcp_client/mgcp_client_pool.h \
+	$(NULL)
+
+noinst_HEADERS = \
 	osmocom/mgcp/mgcp.h \
 	osmocom/mgcp/mgcp_common.h \
 	osmocom/mgcp/osmux.h \