lib: virgo_exec: add string.h
lib: virgo_exec: add string.h

../lib/virgo_exec.c: In function ‘copy_args’:
../lib/virgo_exec.c:40: warning: implicit declaration of function ‘strcmp’
../lib/virgo_exec.c:44: warning: implicit declaration of function ‘strdup’

cp16net/virgo-base,kaustavha/rackspace-monitoring-agent,cp16net/virgo-base,christopherjwang/rackspace-monitoring-agent,cp16net/virgo-base,virgo-agent-toolkit/rackspace-monitoring-agent,kaustavha/rackspace-monitoring-agent,cp16net/virgo-base,AlphaStaxLLC/rackspace-monitoring-agent,virgo-agent-toolkit/rackspace-monitoring-agent,cp16net/virgo-base,AlphaStaxLLC/rackspace-monitoring-agent,christopherjwang/rackspace-monitoring-agent