init/version-timestamp.c: remove unneeded #include <linux/version.h>

The kbuild test robot detected this by 'make versioncheck'.

Fixes: 2df8220c ("kbuild: build init/built-in.a just once")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>