ns: Log when sending fails (Coverity)

Currently the return value of the gprs_ns_tx family of functions is
often ignored. This is not a serious issue, since the successful
delivery of the messages is neither guaranteed nor acknowledged by
the network layer anyway.

Nevertheless this commit adds logging (level INFO) to gprs_ns_tx and
gprs_ns_msgb_alloc. The definition of the latter has been moved from
the header file to gprs_ns.c.

Fixes: Coverity CID 1040678, 1040679, 1040680, 1040681, 1040682,
   1040683, 1040684, 1040686, 1040687, 1040688, 1111545,
   1240203, 1240204
Sponsored-by: On-Waves ehf
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index a21a7ac..43ebbf8 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -57,6 +57,7 @@
 gprs_ns_ll_str;
 gprs_ns_ll_copy;
 gprs_ns_ll_clear;
+gprs_ns_msgb_alloc;
 
 gprs_nsvc_create;
 gprs_nsvc_delete;