Fix compilation with gcc-10

/usr/bin/ld: test_common.o:(.bss+0x0): multiple definition of `talloc_asn1_ctx'; test-helpers.o:(.bss+0x0): first defined here

See also https://alioth-lists.debian.net/pipermail/debian-mobcom-maintainers/Week-of-Mon-20200413/000652.html

Change-Id: I4ce41158bb27e6b9242613106f226fc86995c53c
diff --git a/src/tests/test-helpers.c b/src/tests/test-helpers.c
index 08236fa..6a6a4d1 100644
--- a/src/tests/test-helpers.c
+++ b/src/tests/test-helpers.c
@@ -33,7 +33,7 @@
 #include <osmocom/ranap/RANAP_LAI.h>
 
 int asn1_xer_print = 0;
-void *talloc_asn1_ctx;
+extern void *talloc_asn1_ctx;
 
 /* use odd number of digits */
 const uint8_t imsi_encoded[] = { 0x10, 0x32, 0x54, 0x76, 0xF8 };