mk_signal: Removed unused variable 'old' in function mk_signal_thread_sigpipe_safe()

Removed the sigset_t old from line 71 from function - void mk_signal_thread_sigpipe_safe(). 
It removes the following warning arising during 'make'.
------------------  WARNING  ---------------------
mk_signals.c: In function ‘mk_signal_thread_sigpipe_safe’:
mk_signals.c:71:19: warning: unused variable ‘old’ [-Wunused-variable]
------------------  WARNING  ---------------------