.gitignore: remove stale entry for generated version.h

Since userspace headers were moved to generated/uapi it possible to have a
stale copy of linux/version.h at that file's old location.  This causes
confusion after building an older kernel version, then checking out and
building a new one; the old (stale) version header will still get picked
up until it is manually removed.  This upsets the C library.

Since the uapi changes, include/linux/version.h is no longer generated and
should not be ignored, so this patch removes it from .gitignore.

Signed-off-by: Jonathan Austin <jonathan.austin@arm.com>
Reported-by: Kevin Petit <kevin.petit@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>