Fix problem with undefined umode_t type in 3.3 kernel headers

The 3.3 kernel headers define the umode_t type within a __KERNEL__
preprocessor ifdef region. This results in a broken kernel header in the
buildroot toolchain.

[Peter: This is discussed upstream here:
https://bugzilla.kernel.org/show_bug.cgi?id=42986


Long term socat/syslinux should stop using this header, but this hasn't
been fixed upstream yet]
Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>