libc/minimal: Add stub 'sys/lock.h'

This file is explicitly included by the espressif hal module. It's an
internal file provided by picolibc and newlib. Provide a stub to let
code designed for those to work with the minimal C library.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>