compiler-gcc4: conditionalize #error on __KERNEL__

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: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>