greybus: kernel_ver.h: include <linux/kernel.h> to fix warning

And this is the warning I was getting on kernel version > 3.14

  CC [M]  greybus/connection.o
  In file included from
  include/asm-generic/gpio.h:4:0,
	from arch/arm/include/asm/gpio.h:9,
	from include/linux/gpio.h:48,
	from greybus/kernel_ver.h:59,
	from greybus/connection.c:12:
  include/linux/kernel.h:35:0: warning: "U16_MAX" redefined

kernel_ver.h is taking care of defining U16_MAX only if is not defined earlier,
but it is often included as the first .h file. <linux/kernel.h> might be
included later, which always defines it, unconditionally. And so this warning.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>