Prior to 2a4b472c, sys/signal.h was only included on OpenBSD
(apart from two .c files). The POSIX standard location for this
header is just <signal.h> and in fact, OpenBSD's signal.h includes
sys/signal.h itself.

Unconditionally including <sys/signal.h> on musl causes warnings
for just about every source file:

  /usr/include/sys/signal.h:1:2: warning: #warning redirecting incorrect #include <sys/signal.h> to <signal.h> [-Wcpp]
      1 | #warning redirecting incorrect #include <sys/signal.h> to <signal.h>
        |  ^~~~~~~

Since there don't seem to be any platforms which require including
<sys/signal.h> in addition to <signal.h>, and some platforms like
Haiku lack it completely, just remove it.

Tested building on OpenBSD after removing this include.

Signed-off-by: Michael Forney <mforney@mforney.org>
Tested-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210113215600.16100-1-mforney@mforney.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>