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 <[e-mail]>
Acked-by: Fabrice Gasnier <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/trigger/stm32-timer-trigger.c

···
  *
  */
 
+#include <linux/export.h>
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
 #include <linux/iio/timer/stm32-timer-trigger.h>