package/slirp: add libglib2 mandatory dependency

slirp depends on libglib2, don't update xen as it already depends on it

Fixes:
 - http://autobuild.buildroot.org/results/0b9cff1bc650876a6fff6102b2cb31dcdf4c5e8f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>