autotools: Create src/wycheproof dir before creating file in it

This directory may not exist in a VPATH build,
see bitcoin/bitcoin#27445 (comment) .

Author: real-or-random

Makefile.am

@@ -226,6 +226,7 @@ maintainer-clean-local: maintainer-clean-precomp
 TESTVECTORS = src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.h
 
 src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.h:
+	mkdir -p $(@D)
 	python3 tools/tests_wycheproof_generate.py src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.json > $@
 
 testvectors: $(TESTVECTORS)