MIPS/Loongson: Fix build warnings about export.h

After commit a934a57a ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") and 7d95680d ("scripts/misc-check:
check unnecessary #include <linux/export.h> when W=1"), we get some build
warnings with W=1:

arch/mips/loongson64/setup.c: warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present

So fix these build warnings for MIPS/Loongson.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>