Signals: Removed unused variable 'old' in 'mk_signal_thread_sigpipe_safe()'

Removed the unused variable 'sigset_t old' in line 71 from function, 'void mk_signal_thread_sigpipe_safe()'. This patch removes the following warning caused 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 ---------------------

Signed-off-by: Savita TS <savita.seetaraman5@gmail.com>