zephyr: tests: Add missing header to pwm_led_driver/src/led.c test
zephyr: tests: Add missing header to pwm_led_driver/src/led.c test

This source doesn't include `pwm_mock.h`, causing an error when
compiling with GCC (LLVM toolchain in chroot doesn't care) because
`pwm_mock_get_duty` is implicitly defined.

BUG=None
BRANCH=None
TEST=./twister and Gitlab CI in Docker

Signed-off-by: Tristan Honscheid <1e17951ff5ffc8afe254954c03de64afef62fe52@google.com>
Change-Id: Ic1d458956832af7420506f822ea0e5ec2faae96a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3846998
Commit-Queue: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@google.com>
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@google.com>
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec