fluxbox: fix build against xlib_libXpm

Explicitly configure XPM support based on the library selection provided
(BR2_PACKAGE_XLIB_LIBXPM). It has been observed on different hosts that
XPM support may or may not be included based off the state of the host's
system. One can validate XPM support is included in Fluxbox by using the
command `fluxbox -info`.

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>