[GPRS] Initial untested support for libgtp

libgtp of the OpenGGSN project will allow us to speak the GTPv0/v1
protocol of the interface between SGSN and GGSN.

This commit includes code for the main libgtp integration (file
descriptor, select loop, timer) as well as code to encode/send
a CREATE PDP CONTEXT request.
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index ba46719..90ede76 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -81,8 +81,9 @@
 struct sgsn_mm_ctx *sgsn_mm_ctx_alloc(uint32_t tlli,
 					const struct gprs_ra_id *raid)
 {
-	struct sgsn_mm_ctx *ctx = talloc_zero(NULL, struct sgsn_mm_ctx);
+	struct sgsn_mm_ctx *ctx;
 
+	ctx = talloc_zero(tall_bsc_ctx, struct sgsn_mm_ctx);
 	if (!ctx)
 		return NULL;