requirements.txt: add gcovr
requirements.txt: add gcovr

This tool is cited in our documentation for producing coverage
reports, add it to the list of packages pulled in by pip during
workstation setup.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr