include: posix: correcting the return type of sleep.
include: posix: correcting the return type of sleep.

Currently return type of sleep is int, but as per POSIX 1003.1 it
should be unsigned.

Signed-off-by: Youvedeep Singh <7f33386d52951c3e7a699b9509267436d9be79f8@intel.com>

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