GUFI: remove extra checks for CONFIG_GUFI

It's not needed to wrap each file in #ifdef CONFIG_GUFI, because we already
choose it via obj-$(CONFIG_GUFI) in Makefile.

Signed-off-by: Semen Protsenko <semen.protsenko@linaro.org>
GUFI: remove extra "extern" specifiers

No need in making functions "extern" in header, all functions are already
act as "extern".

Signed-off-by: Semen Protsenko <semen.protsenko@linaro.org>