userspace: eliminate warning in process_gperf.py
userspace: eliminate warning in process_gperf.py

Python 3.7 throws a FutureWarning when it thinks it
has found a nested set in a regular expression.

We weren't actually; just trying to match some brackets.
Escape them instead. No functional difference.

Fixes: #11961

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

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