libraw: add dependency on host-pkgconf

Otherwise, it fails to autoreconf with a weird error message:

configure.ac:15: error: possibly undefined macro: AC_SUBST
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
configure.ac:40: error: possibly undefined macro: AC_MSG_WARN

Indeed, the configure.ac uses PKG_CHECK_MODULES(), so we need to have
the corresponding m4 file installed by host-pkgconf.

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