package/x11r7/xapp_bdftopcf: Fix linking error

Fixes
http://autobuild.buildroot.net/results/242/24297c948c0eee684e868ad4bd12f11c3a759248/
http://autobuild.buildroot.net/results/464/464fdc71690a437c1782dd8e4c75d78195bf8bfd/
http://autobuild.buildroot.net/results/77c/77c26d14c8906b44f7c3fa1b3d759556c7318a7e/
and many others ( http://autobuild.buildroot.net/?reason=xapp_bdftopcf-1.0.4

 )

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>