xfwm4: add exo-native to DEPENDS

fixes (detected in Martin's world build):

| make[2]: Entering directory `/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/work/armv5te-oe-linux-gnueabi/xfwm4/4.10.1+gitAUTOINC+bce3062d08-r0/git/settings-dialogs'
| gdk-pixbuf-csource --raw --build-list \
| 	monitor_icon_data ./monitor-icon.png > monitor-icon.h
| exo-csource --static --strip-comments --strip-content --name=xfwm4_dialog_ui xfwm4-dialog.glade >xfwm4-dialog_ui.h
| exo-csource --static --strip-comments --strip-content --name=tweaks_dialog_ui xfwm4-tweaks-dialog.glade >xfwm4-tweaks-dialog_ui.h
| exo-csource --static --strip-comments --strip-content --name=workspace_dialog_ui xfwm4-workspace-dialog.glade >xfwm4-workspace-dialog_ui.h
| /bin/bash: exo-csource: command not found
| /bin/bash: exo-csource: command not found
| /bin/bash: exo-csource: command not found
| make[2]: *** [xfwm4-dialog_ui.h] Error 127
| make[2]: *** Waiting for unfinished jobs....
| make[2]: *** [xfwm4-tweaks-dialog_ui.h] Error 127
| make[2]: *** [xfwm4-workspace-dialog_ui.h] Error 127
| make[2]: Leaving directory `/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/work/armv5te-oe-linux-gnueabi/xfwm4/4.10.1+gitAUTOINC+bce3062d08-r0/git/settings-dialogs'
| make[1]: *** [all-recursive] Error 1
| make[1]: Leaving directory `/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/work/armv5te-oe-linux-gnueabi/xfwm4/4.10.1+gitAUTOINC+bce3062d08-r0/git'
| make: *** [all] Error 2
| ERROR: oe_runmake failed
| WARNING: exit code 1 from a shell command.
| ERROR: Function failed: do_compile (log file is located at /home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/work/armv5te-oe-linux-gnueabi/xfwm4

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>