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

This is a follow-up to commit e765f8fab where we made this header safe
to include, but a no-op on Windows.

Review: https://reviews.apache.org/r/67532/

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