Removed superfluous guard around `signals.hpp`.
Removed superfluous guard around `signals.hpp`.

Review: https://reviews.apache.org/r/67533/

abudnik/mesos,dforsyth/mesos,andschwa/mesos,dforsyth/mesos,andschwa/mesos,andschwa/mesos,dforsyth/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,chhsia0/mesos,abudnik/mesos,jpeach/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,kaysoky/mesos,gsantovena/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,jpeach/mesos,andschwa/mesos,andschwa/mesos,Gilbert88/mesos,chhsia0/mesos,chhsia0/mesos,jpeach/mesos,abudnik/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,kaysoky/mesos,kaysoky/mesos,andschwa/mesos,abudnik/mesos,gsantovena/mesos,dforsyth/mesos,jpeach/mesos