Makefile: Remove more pkgconfig files

The pkgconfig files are located in  /usr/lib/pkgconfig
and /usr/share/pkgconfig directories.
However, only /usr/lib/pkgconfig is removed when no
development files are needed in the target filesystem.

Remove pkgconfig directory from $(TARGET_DIR)/usr/share
as well if BR2_HAVE_DEVFILES is not set.

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>