tests/tests_common.c: Add missing header

The header is probably indirectly added by osmocom/iuh/hnbgw.h, but once
that's dropped, really weird stuff happens, like talloc crashing in
talloc_total_blocks() because the compiled code incorrectly fills
msgb_ctx variable in return of msgb_talloc_ctx_init.

Change-Id: I7dab8d18d316632989406a9f22917e00a7fd68bf

src/tests/test_common.c

@@ -36,6 +36,7 @@
 #include <osmocom/core/application.h>
 #include <osmocom/core/talloc.h>
 #include <osmocom/core/logging.h>
+#include <osmocom/core/msgb.h>
 
 #include <osmocom/ranap/ranap_common.h>