include: arch: ffs: include toolchain/common.h

Add an include of toolchain/common.h for the ALWAYS_INLINE
macro.

This is a general issue in many files across Zephyr, specifically
for the ALWAYS_INLINE macro, but for simplicity this commit
only includes it for this file as that has been shown to
cause compilaion issues without it.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>