libc/common: Don't use malloc mutex when CONFIG_MULTITHREADING=n

When multithreading is disabled, the whole mutex infrastructure isn't
available. The common malloc code wasn't checking for this case which
caused build failures.

Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Detlev Zundel <dzu@member.fsf.org>