spi: loongson: Fix build warnings about export.h

After commit a934a57 ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") and 7d95680 ("scripts/misc-check:
check unnecessary #include <linux/export.h> when W=1"), we get some build
warnings with W=1:

drivers/spi/spi-loongson-core.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

So fix these build warnings for SPI/Loongson.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/20250608142939.172108-1-chenhuacai@loongson.cn
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: chenhuacai

drivers/spi/spi-loongson-core.c

@@ -5,6 +5,7 @@
 #include <linux/clk.h>
 #include <linux/delay.h>
 #include <linux/err.h>
+#include <linux/export.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/io.h>