drop code dup between libosmo-mgcp-client and libosmo-mgcp

Remove mgcp_common.c and replace with mgcp_common.h.

Move mgcp_common.h from mgcp_client/ to mgcp/;
Place a compile-time copy of it back in mgcp_client/.

Add builddir/include to compiler -I paths to find generated header.

Rationale:
- Keep separate copies of the file for each of the library to not require
  debian dependencies between the two libraries.
- Avoid code dup by copying during 'make' (think: a generated header, BUILT_SOURCE).
- The copy does not have implications for linking (like mgcp_common.c did) nor
  is it a source for build confusion or fallout in other projects, because it
  does not reach across several git source trees (like gsm_data_shared.h did).

mgcp_connection_mode_strs are not actually used in libosmo-mgcp, so drop them.
(It would make semantic sense to have then in mgcp, but we can add it when it
is needed. A similar value string array remains in libosmo-mgcp-client.)

Change-Id: I7a5d3b9a2eb90be7e34b95efa529429f2e6c3ed8
diff --git a/src/libosmo-mgcp/mgcp_network.c b/src/libosmo-mgcp/mgcp_network.c
index 4a1eb74..dac1698 100644
--- a/src/libosmo-mgcp/mgcp_network.c
+++ b/src/libosmo-mgcp/mgcp_network.c
@@ -34,6 +34,7 @@
 #include <osmocom/core/select.h>
 #include <osmocom/netif/rtp.h>
 #include <osmocom/mgcp/mgcp.h>
+#include <osmocom/mgcp/mgcp_common.h>
 #include <osmocom/mgcp/mgcp_internal.h>
 #include <osmocom/mgcp/mgcp_stat.h>
 #include <osmocom/mgcp/osmux.h>