mgcp: Move the SDP handling into a separate file/module

The SDP file handling will get more complicated in terms of
codec selection so let's remove it from the protocol handling
before we start blowing it up in size.
diff --git a/openbsc/src/libmgcp/Makefile.am b/openbsc/src/libmgcp/Makefile.am
index d02b880..4403d60 100644
--- a/openbsc/src/libmgcp/Makefile.am
+++ b/openbsc/src/libmgcp/Makefile.am
@@ -8,7 +8,8 @@
 
 noinst_HEADERS = g711common.h
 
-libmgcp_a_SOURCES = mgcp_protocol.c mgcp_network.c mgcp_vty.c mgcp_osmux.c
+libmgcp_a_SOURCES = mgcp_protocol.c mgcp_network.c mgcp_vty.c mgcp_osmux.c \
+	mgcp_sdp.c
 
 if BUILD_MGCP_TRANSCODING
     libmgcp_a_SOURCES += mgcp_transcode.c