package/openocd: add missing host-pkgconf dependency

The configure.ac script uses PKG_CHECK_MODULES(), and we autoreconf
the package, so host-pkgconf should be listed in the dependencies.

This issue is seen either with per-package folders, or by doing a
clean build with just "make openocd":

>>> openocd 0.10.0 Configuring
>>> openocd 0.10.0 Autoreconfiguring
[...]
configure.ac:12: error: possibly undefined macro: AC_MSG_WARN
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
configure.ac:201: error: possibly undefined macro: AC_DEFINE
configure.ac:582: error: possibly undefined macro: AC_MSG_NOTICE

Even if the message seems unrelated, it's really the lack of pkg.m4
from host-pkgconf that causes the issue.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>