contrib: libressl: Validate checksum and fix prerequisite.

The 'fetch' target relies on the prerequisites of the .sum-$(PKG)
files.  Prior to this change, 'make fetch' would not fetch the
openbsd-libressl tarball.

Change-Id: Ic91cc7032243d0c0f3b3d69fb74cfb3c633d0fc3

contrib/src/libressl/SHA512SUMS

 04761ea886968ce810c0555f8251289811bb980bd58629aae5084d289173295bf68fb7a997dc98f290cabe0edbe31c277d09023158cee81aa35d98ce49ceac52  portable-v3.2.1.tar.gz
+57cda84144534a6da1db815a57fc85a39f5e2c3eaa085f489ecfdcac6fc72ec8156157875680aed9c850153e15d3cfbb4202de36b9e7218d662cbbed965d42cb  openbsd-libressl-v3.2.1.tar.gz

contrib/src/libressl/rules.mak

@@ -63,4 +63,4 @@ endif
 	rm -rf $(PREFIX)/lib/*.so $(PREFIX)/lib/*.so.*
 	touch $@
 
-.sum-libressl: portable-$(LIBRESSL_VERSION).tar.gz
+.sum-libressl: portable-$(LIBRESSL_VERSION).tar.gz openbsd-$(OPENBSD_VERSION).tar.gz