xfce4-screenshooter: Ensure lib directory exists.

Screenshooter autogenerates screenshooter-marshal.[ch] in lib/ during
compilation.  The source directory contains a lib/ but this directory
doesn't exist in a build directory by default so we add one manually.
Really, this should be corrected in the upstream package.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>