Implement GSMTAP log target

This target wraps the to-be-logged string (With metadata) into a GSMTAP
packet and sends it to the configured destination address.

Change-Id: I9a7e72b8c9c6f6f2d76d1ea2332dcdee12394625
diff --git a/src/logging.c b/src/logging.c
index 165d822..1516a7d 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -240,7 +240,7 @@
 	return NULL;
 }
 
-static const char* category_name(int subsys)
+const char* log_category_name(int subsys)
 {
 	if (subsys < osmo_log_info->num_cat)
 		return osmo_log_info->cat[subsys].name;
@@ -290,7 +290,7 @@
 			OSMO_SNPRINTF_RET(ret, rem, offset, len);
 		}
 		if (target->print_category) {
-			ret = snprintf(buf + offset, rem, "%s ", category_name(subsys));
+			ret = snprintf(buf + offset, rem, "%s ", log_category_name(subsys));
 			if (ret < 0)
 				goto err;
 			OSMO_SNPRINTF_RET(ret, rem, offset, len);