package/x11r7/xlib_libICE: Add host-pkgconf dependency

Using this defconfig

BR2_x86_pentium4=y
BR2_COMPILER_PARANOID_UNSAFE_PATH=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-i386-pentium4-full-2015.02.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_2=y
BR2_TOOLCHAIN_EXTERNAL_INET_IPV6=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_INET_RPC=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XLIB_LIBICE=y

build would fail without this patch:

checking for ICE... configure: error: in `/home/br/buildroot/output/build/xlib_libICE-1.0.9':
configure: error: The pkg-config script could not be found or is too old.  Make sure it
is in your PATH or set the PKG_CONFIG environment variable to the full
path to pkg-config.

Alternatively, you may set the environment variables ICE_CFLAGS
and ICE_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

To get pkg-config, see <http://pkg-config.freedesktop.org/

>.
See `config.log' for more details

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>