syscall: Z_SYSCALL_VERIFY_MSG to ignore -Wshadow

Z_SYSCALL_VERIFY_MSG() does LOG_MODULE_DECLARE() internally to
log errors during verification. However, this is definitely
going to conflict with any LOG_MODULE_DECLARE() in the source
file using Z_SYSCALL_VERIFY_MSG(). So we need to selectively
disable -Wshadow for Z_SYSCALL_VERIFY_MSG() to avoid compiler
warnings as this is an intentional behavior.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>