package/snappy: add dependency on host-pkgconf

Snappy doesn't configure without host pkg-config, causing this totally
unhelpful diagnostic from autoconf:

  configure.ac:42: error: possibly undefined macro: AC_DEFINE
        If this token and others are legitimate, please use m4_pattern_allow.
        See the Autoconf documentation.
  configure.ac:44: error: possibly undefined macro: AC_MSG_FAILURE

So add host-pkgconf to the package's DEPENDENCIES list.

Signed-off-by: Steve James <ste@junkomatic.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>