Add libpq's openssl dependencies to pkg-config file

Add libssl and libcrypto to libpq.pc's Requires.private.  This allows
static linking to work if those libssl or libcrypto themselves have
dependencies in their *.private fields, such as -lz in some cases.
Reported-by: Sandro Mani <manisandro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/837d1dcf-2fca-ee6e-0d7e-6bce1a1bac75@gmail.com

src/interfaces/libpq/Makefile

@@ -89,6 +89,8 @@ SHLIB_PREREQS = submake-libpgport
 
 SHLIB_EXPORTS = exports.txt
 
+PKG_CONFIG_REQUIRES_PRIVATE = libssl libcrypto
+
 all: all-lib
 
 # Shared library stuff