ASoC: loongson: Fix build warnings about export.h

After commit 25704938c86d7b2 ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") and 1239f681359926101e ("scripts/misc-check:
check unnecessary #include <linux/export.h> when W=1"), we get some build
warnings with W=1:

sound/soc/loongson/loongson_i2s.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

So fix these build warnings for ASoC/Loongson.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/20250608142737.168829-1-chenhuacai@loongson.cn


Signed-off-by: Mark Brown <broonie@kernel.org>