kasumi: Fix build with gcc5 that changed behavior of inline

GCC 5 has changed inline semantics (https://gcc.gnu.org/gcc-5/porting_to.html)
and apparently that now fails to link.
diff --git a/tests/kasumi/kasumi_test.c b/tests/kasumi/kasumi_test.c
index 9101407..a4b1f8d 100644
--- a/tests/kasumi/kasumi_test.c
+++ b/tests/kasumi/kasumi_test.c
@@ -10,7 +10,7 @@
 
 /* Test vectors are taken from TS 135 202 */
 
-inline int _compare_mem(uint8_t * x, uint8_t * y, size_t len)
+inline static int _compare_mem(uint8_t * x, uint8_t * y, size_t len)
 {
 	if (0 != memcmp(x, y, len)) {
 		printf ("X: %s\t", osmo_hexdump_nospc(x, len));