libsoc: add host-pkgconf dependency

libsoc: add host-pkgconf dependency

Without pkg-config PKG_CHECK_MODULES won't be expanded
and ./configure script produces following error message:

./configure: line 12237: syntax error near unexpected token `PYTHON,'
./configure: line 12237: `	PKG_CHECK_MODULES(PYTHON, python-"$PYTHON_VERSION")'

Fixes:

http://autobuild.buildroot.net/results/86e/86e04bd2b10527130306451e56a7693ed4b4befd

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