build: Add missing pkg-config Requires.private dependencies

Otherwise, a trivial executable that includes <act/act.h>, built with $(pkg-config --cflags --libs accountsservice), will fail to compile when we cannot include headers like <glib-object.h>.

---

Test-case:

#!/bin/sh
# Copyright 2020 Collabora Ltd.
# SPDX-License-Identifier: GPL-2.0-or-later

set -eux

if [ -n "${DEB_HOST_GNU_TYPE:-}" ]; then
    CROSS_COMPILE="$DEB_HOST_GNU_TYPE-"
else
    CROSS_COMPILE=
fi

cat > trivial.c <<EOF
#include <act/act.h>

int main(void)
{
    g_assert_cmpint (ACT_TYPE_USER_MANAGER, !=, 0);
    return 0;
}
EOF

# Deliberately word-splitting pkg-config's output:
# shellcheck disable=SC2046
"${CROSS_COMPILE}gcc" -otrivial trivial.c $("${CROSS_COMPILE}pkg-config" --cflags --libs accountsservice glib-2.0)
./trivial