net/libmdf: Add missing openssl library argument

The build relies on pkg-config to supply "-lcrypto" but we're using
OPENSSL_CFLAGS and OPENSSL_LIBS (not pkg-config), to enable libmdf to find
openssl in base, which does not provide an openssl.pc file.

The previous commit neglected to explicitly include the library to link with
(-lcrypto), resulting in undefined references to OpenSSL functions when
attempting to link with the libmdf library.

PR: 219777
Reported by: Henrik Holst <henrik.holst millistream com>