gtphub_test: Fix compilation with gcc-10

/usr/bin/ld: ../../src/gtphub/gtphub.o:/home/laforge/projects/git/osmo-sgsn/src/gtphub/gtphub.c:50: multiple definition of `osmo_gtphub_ctx'; gtphub_test.o:/home/laforge/projects/git/osmo-sgsn/tests/gtphub/gtphub_test.c:57: first defined here
collect2: error: ld returned 1 exit status

See also https://alioth-lists.debian.net/pipermail/debian-mobcom-maintainers/Week-of-Mon-20200413/000653.html

Change-Id: I19c1eef6649d2747f0b624f5292d7ae47c4ca839
diff --git a/tests/gtphub/gtphub_test.c b/tests/gtphub/gtphub_test.c
index 2e48bb1..ed46176 100644
--- a/tests/gtphub/gtphub_test.c
+++ b/tests/gtphub/gtphub_test.c
@@ -54,7 +54,7 @@
 void gtphub_init(struct gtphub *hub);
 void gtphub_free(struct gtphub *hub);
 
-void *osmo_gtphub_ctx;
+extern void *osmo_gtphub_ctx;
 
 static void nr_mapping_free(struct expiring_item *e)
 {