pmsignal.h uses sig_atomic_t in some builds, but relied on signal.h
having been included already.  We could include it conditionally
but evidently that wouldn't save anything in practice and would
add more ugly macros, so let's just include signal.h always.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/4166.1533154074%40sss.pgh.pa.us