secp-sys: update scratch_impl.patch for upstream changes

Author: apoelstra

secp256k1-sys/depend/scratch_impl.h.patch

@@ -15,7 +15,7 @@
 < static void secp256k1_scratch_destroy(const secp256k1_callback* error_callback, secp256k1_scratch* scratch) {
 <     if (scratch != NULL) {
 <         VERIFY_CHECK(scratch->alloc_size == 0); /* all checkpoints should be applied */
-<         if (memcmp(scratch->magic, "scratch", 8) != 0) {
+<         if (secp256k1_memcmp_var(scratch->magic, "scratch", 8) != 0) {
 <             secp256k1_callback_call(error_callback, "invalid scratch space");
 <             return;
 <         }