mgcp_e1: finish E1 support, add E1 support from libosmoabis

Currently only the endpoint handling for E1 exists, but there is no
actual code behind it that handles the E1 traffic.

Change-Id: I6b93809b5ac7d01af55888347dd787b0bc997ae1
Related: OS#2659
diff --git a/include/osmocom/mgcp/Makefile.am b/include/osmocom/mgcp/Makefile.am
index fb7654f..549ba87 100644
--- a/include/osmocom/mgcp/Makefile.am
+++ b/include/osmocom/mgcp/Makefile.am
@@ -10,4 +10,5 @@
 	mgcp_trunk.h \
 	debug.h \
 	mgcp_ratectr.h \
+	mgcp_e1.h \
 	$(NULL)