iio: trigger: stm32-timer: Fix build warnings about export.h

iio: trigger: stm32-timer: Fix build warnings about export.h

After commit a934a57a42f6 ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") and commit 7d95680d64ac ("scripts/misc-check:
check unnecessary #include <linux/export.h> when W=1") we get the build
warning with W=1:

drivers/iio/trigger/stm32-timer-trigger.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

Fix it.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://patch.msgid.link/20250610124855.269158-4-antonio.borneo@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>