mocking: Update fff.h
mocking: Update fff.h

Update fff.h to the pending PR upstream which allows for using a custom
function signature. This enables the use of C++ std::function as the
mock.

Signed-off-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@google.com>

zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr