Avoids a missing brace warning in schnorrsig/tests_impl.h on old compilers.

GCC 4.9.2, at least, emits "warning: missing braces around initializer"
 without this.

src/modules/schnorrsig/tests_impl.h

@@ -103,7 +103,7 @@ void test_schnorrsig_api(void) {
     unsigned char sk3[32];
     unsigned char msg[32];
     secp256k1_keypair keypairs[3];
-    secp256k1_keypair invalid_keypair = { 0 };
+    secp256k1_keypair invalid_keypair = {{ 0 }};
     secp256k1_xonly_pubkey pk[3];
     secp256k1_xonly_pubkey zero_pk;
     unsigned char sig[64];