mfd: stm32-timers: 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/mfd/stm32-timers.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://lore.kernel.org/r/20250610124855.269158-6-antonio.borneo@foss.st.com
Signed-off-by: Lee Jones <lee@kernel.org>