LoongArch: 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/loongarch/kernel/acpi.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/alternative.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/kfpu.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/traps.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/unwind_guess.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/unwind_orc.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/unwind_prologue.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/lib/crc32-loongarch.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/lib/csum.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
arch/loongarch/kernel/elf.c: warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present
arch/loongarch/kernel/paravirt.c: warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present
arch/loongarch/pci/pci.c: warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present

So fix these build warnings for LoongArch.

Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>