xfce4-screenshooter: ensure output directory is created

When build with B != S, the output directory for desktop
file doesn't exist so it will fail if it's not created
before generating the file:
/bin/sh: line 1: src/xfce4-screenshooter.desktop.in: No such file or directory

Add "$(MKDIR_P) $(dir $@)" to the rule to fix this.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>