keyword-speech: Fix serial terminal race condition issues

Adding mutex to be used for resolving race condition between
the ml task ml  which prints the inference result and other tasks
in which the race condition produce corrupted print statements
on the serial terminal which lead to unicode decode error
when trying to read the statements on the serial terminal
by the python test applications.

Signed-off-by: Ahmed.Ismail <Ahmed.Ismail@arm.com>