lib/util: Add signal.h include
Fixes build error with samba-4.15.3 and uClibc:
../../source3/printing/samba-bgqd.c: In function ‘main’:
../../source3/printing/samba-bgqd.c:340:21: error: ‘SIGPIPE’ undeclared (first use in this function); did you mean ‘EPIPE’?
../../source3/printing/samba-bgqd.c:384:14: error: ‘SIGTERM’ undeclared (first use in this function)
Checklist
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Commits have Signed-off-by:with name/author being identical to the commit author