libpq: Fix pkg-config without OpenSSL

Do not add OpenSSL dependencies to libpq pkg-config file if OpenSSL is
not enabled.  Oversight in beff361b.

Author: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/20220331163759.32665-1-fontaine.fabrice%40gmail.com

src/interfaces/libpq/Makefile

@@ -94,7 +94,9 @@ SHLIB_PREREQS = submake-libpgport
 
 SHLIB_EXPORTS = exports.txt
 
+ifeq ($(with_ssl),openssl)
 PKG_CONFIG_REQUIRES_PRIVATE = libssl libcrypto
+endif
 
 all: all-lib