x86/vgtod: Remove unused typedef gtod_long_t

The typedef gtod_long_t is not used anymore so remove it.

The header file contains then only includes dependent on
CONFIG_GENERIC_GETTIMEOFDAY to not break ARCH=um. Nevertheless, keep the
header file only with those includes to prevent spreading ifdeffery all
over the place.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>