procps-ng: add dependency on host-pkgconf

procps-ng configure.ac script uses PKG_CHECK_MODULES(), which needs
pkg-config. Until now, it wasn't mandatory since for the ncurses check
it tries PKG_CHECK_MODULES() and then falls back to AC_CHECK_LIB(),
but for the upcoming systemd support, pkg-config is really needed.

Since pkg-config is used both for the ncurses and systemd checks, we
make it a mandatory dependency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>