Impact: Fix for exported headers

We only want to error out on specific gcc versions if we are actually
building the kernel, so conditionalize the #if...#error on __KERNEL__.

Based on a patchset by Arnd Bergmann <arnd@arndb.de>.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>