syscalls: REVERTME: clean up warnings when building unit tests
syscalls: REVERTME: clean up warnings when building unit tests

This is a temporary hack until #5006 is resolved (possibly using

https://github.com/zephyrproject-rtos/zephyr/issues/5006

Unit testing (BOARD == unit_testing) doesn't need the system call
definitions. Because we foward declare with __syscall them as "static
inline" (from common.h), the compilers will complain that the
definition is missing.

Change to only define __syscall as "static inline" if we are not
builing a unit test to avoid said warnings.

Signed-off-by: Inaky Perez-Gonzalez <96af2f95d738a33e3cb28686ea74cc090643af9b@intel.com>

kraj/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyriot/zephyr,finikorg/zephyr,punitvara/zephyr,mbolivar/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,ldts/zephyr,zephyriot/zephyr,explora26/zephyr,Vudentz/zephyr,kraj/zephyr,zephyriot/zephyr,mbolivar/zephyr,finikorg/zephyr,kraj/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,explora26/zephyr,finikorg/zephyr,mbolivar/zephyr,ldts/zephyr,galak/zephyr,zephyriot/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,punitvara/zephyr,galak/zephyr,kraj/zephyr,nashif/zephyr