package/matchbox: add missing expat dependency

package/matchbox: add missing expat dependency

matchbox selects expat but does not add it to its dependency resulting
in the following build failure:

configure: error: cannot find expat library
make: *** [/home/buildroot/autobuild/run/instance-2/output-1/build/matchbox-1.2/.stamp_configured] Error 1

Fixes:
 - http://autobuild.buildroot.org/results/37021f1d7fcfd890011068a28ce6181dc509e746

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>