contrib: fix secp256k1 build on iOS

Fix x86_64 build on Apple silicon.

Change-Id: I233a1e4f48d52a5f9dc56090c864c957a69a2221

contrib/src/secp256k1/rules.mak

@@ -18,7 +18,7 @@ secp256k1: secp256k1-$(SECP256K1_VERSION).tar.gz
 	$(UNPACK)
 	$(MOVE)
 
-ifeq ($(IOS_TARGET_PLATFORM),iPhoneOS)
+ifdef HAVE_IOS
 SECP256K1_CFLAGS := -USECP256K1_BUILD $(CFLAGS)
 else
 SECP256K1_CFLAGS := -USECP256K1_BUILD