make-test: Fix the vapi test

Change-Id: I5a4dc3894818f92368d6c906ff43e093c16c1d19
Signed-off-by: Mohsin Kazmi <sykazmi@cisco.com>
diff --git a/test/ext/vapi_c_test.c b/test/ext/vapi_c_test.c
index e545717..9fbb2a6 100644
--- a/test/ext/vapi_c_test.c
+++ b/test/ext/vapi_c_test.c
@@ -22,6 +22,7 @@
 #include <assert.h>
 #include <setjmp.h>
 #include <check.h>
+#include <vppinfra/string.h>
 #include <vapi/vapi.h>
 #include <vapi/vpe.api.vapi.h>
 #include <vapi/interface.api.vapi.h>