package/x11r7/xapp_rgb: add missing host-pkgconf dependency

package/x11r7/xapp_rgb: add missing host-pkgconf dependency

Building a minimal defconfig such as:

BR2_arm=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-arm-full-2018.05.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_4_9=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_1=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_INIT_NONE=y
BR2_SYSTEM_BIN_SH_NONE=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XAPP_RGB=y

by running "make xapp_rgb" gives the following build failure:

checking for RGB... configure: error: in `/home/test/buildroot/output/build/xapp_rgb-1.0.6':
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 RGB_CFLAGS
and RGB_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

The configure script uses pkg-config, but host-pkgconf is missing in
the list of dependencies.

This issue was detected thanks to per-package directory support.

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