genxml: require future imports for python2 compatibility.
genxml: require future imports for python2 compatibility.

Signed-off-by: Dylan Baker <c26a678a04c601e0311b0d6006e67eee6ed19a8e@intel.com>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>
cc: 12.0 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler