package/qt5/qt5base: fix PKG_CONFIG_LIBDIR

By default pkgconf searches "${libdir}/pkgconfig:${datadir}/pkgconfig"
for .pc files:
https://github.com/pkgconf/pkgconf/blob/844f0a66621629a65e3cbf3c3ca47185602e9df5/configure.ac#L41

The new xorgproto package installs .pc files to ${datadir}/pkgconfig
whichs translates to $(STAGING_DIR)/usr/share/pkgconfig, for example
dri3proto:
https://cgit.freedesktop.org/xorg/proto/xorgproto/tree/Makefile.am#n65

whereas the old xproto_ packages installed their .pc files to
${libdir}/pkgconfig:
https://cgit.freedesktop.org/xorg/proto/dri3proto/tree/Makefile.am?id=3937f72040b79c7245b261da880364177cc40a6e#n4

Normally this upstream change is not a problem because pkgconf searches
both paths for .pc files, unless we define PKG_CONFIG_LIBDIR with
something else like we did for the last six years ;)
https://github.com/buildroot/buildroot/blame/master/package/qt5/qt5base/qt5base.mk#L280

This patch removes all variables concerning pkgconf paths. At the time
the qt5base package was introduced the pkgconf wrapper did not exist.
Today these variables are not necessary anymore.

Fixes
http://autobuild.buildroot.net/results/061/06159500c27026d56a772fafbb0d956c72dd4e6c/



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