freebsd: Make libgb compile on FreeBSD

Include header files for recfrom/AF_INET and include a
struct ip_hdr as it is not available on *BSD.
diff --git a/src/gb/gprs_ns.c b/src/gb/gprs_ns.c
index cdee833..aa7ff93 100644
--- a/src/gb/gprs_ns.c
+++ b/src/gb/gprs_ns.c
@@ -66,6 +66,8 @@
 #include <errno.h>
 #include <stdint.h>
 
+#include <sys/types.h>
+#include <sys/socket.h>
 #include <arpa/inet.h>
 
 #include <osmocom/core/msgb.h>