action-button: link with x11 library
It fixes the following warnings during Debian package build:
dpkg-shlibdeps: warning: debian/gnome-panel/usr/lib/gnome-panel/modules/org.gnome.gnome-panel.action-button.so contains an unresolvable reference to symbol XQueryTree: it's probably a plugin
dpkg-shlibdeps: warning: debian/gnome-panel/usr/lib/gnome-panel/modules/org.gnome.gnome-panel.action-button.so contains an unresolvable reference to symbol XKeysymToKeycode: it's probably a plugin
dpkg-shlibdeps: warning: debian/gnome-panel/usr/lib/gnome-panel/modules/org.gnome.gnome-panel.action-button.so contains an unresolvable reference to symbol XFree: it's probably a plugin
dpkg-shlibdeps: warning: debian/gnome-panel/usr/lib/gnome-panel/modules/org.gnome.gnome-panel.action-button.so contains an unresolvable reference to symbol XKillClient: it's probably a plugin
dpkg-shlibdeps: warning: debian/gnome-panel/usr/lib/gnome-panel/modules/org.gnome.gnome-panel.action-button.so contains an unresolvable reference to symbol XInternAtom: it's probably a plugin
dpkg-shlibdeps: warning: debian/gnome-panel/usr/lib/gnome-panel/modules/org.gnome.gnome-panel.action-button.so contains an unresolvable reference to symbol XGetWindowProperty: it's probably a plugin