fix various compiler warnings (on FreeBSD-11.0)

FreeBSD 11.0 uses clang version 3.8.0 which spits various warnings
during libosmocore compilation.  Let's clean this up a bit.

Change-Id: Ic14572e6970bd0b8916604fabf807f1608fa07e5
diff --git a/src/gsm/ipa.c b/src/gsm/ipa.c
index 6dc3ab6..f44c328 100644
--- a/src/gsm/ipa.c
+++ b/src/gsm/ipa.c
@@ -28,6 +28,7 @@
 
 #include <arpa/inet.h>
 #include <sys/types.h>
+#include <sys/socket.h>
 
 #include <osmocom/core/msgb.h>
 #include <osmocom/core/talloc.h>