osmux: rename osmux.c to mgcp_osmux.c

So we can easily identify in the log message what refers to
libosmo-netif and what to libmgcp.
diff --git a/openbsc/src/libmgcp/Makefile.am b/openbsc/src/libmgcp/Makefile.am
index e5dab1a..d02b880 100644
--- a/openbsc/src/libmgcp/Makefile.am
+++ b/openbsc/src/libmgcp/Makefile.am
@@ -8,7 +8,7 @@
 
 noinst_HEADERS = g711common.h
 
-libmgcp_a_SOURCES = mgcp_protocol.c mgcp_network.c mgcp_vty.c osmux.c
+libmgcp_a_SOURCES = mgcp_protocol.c mgcp_network.c mgcp_vty.c mgcp_osmux.c
 
 if BUILD_MGCP_TRANSCODING
     libmgcp_a_SOURCES += mgcp_transcode.c