kbuild: move warnings about linux/export.h from W=1 to W=2

This hides excessive warnings, as nobody builds with W=2.

Fixes: a934a57a ("scripts/misc-check: check missing #include <linux/export.h> when W=1")
Fixes: 7d95680d ("scripts/misc-check: check unnecessary #include <linux/export.h> when W=1")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>