xfsprogs: define umode_t for build if not defined already

xfsprogs: define umode_t for build if not defined already

umode_t has not been exported to the kernel private headers since
around kernel 3.2.0-rc7.  Add a check for umode_t and define it
if it has not been defined already.

Signed-off-by: Michael L. Semon <mlsemon35@gmail.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>