Fix build.

Forgot to mark something static.

Change-Id: I497075d0ad27e2062f84528fb568b333e72a7d3b
Reviewed-on: https://boringssl-review.googlesource.com/7753
Reviewed-by: David Benjamin <davidben@google.com>

crypto/obj/obj_test.cc

@@ -68,7 +68,7 @@ static bool TestBasic() {
   return true;
 }
 
-bool TestSignatureAlgorithms() {
+static bool TestSignatureAlgorithms() {
   int digest_nid, pkey_nid;
   if (!OBJ_find_sigid_algs(NID_sha256WithRSAEncryption, &digest_nid,
                            &pkey_nid) ||