replace signal.h with csignal.h

The former was deprecated with C++14. The standard says they are the same:

The contents of the header<csignal>are the same as the C standard library
header<signal.h>.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/event/SignalMonitor.cxx

@@ -38,13 +38,12 @@
 
 #include <algorithm>
 #include <cassert>
+#include <csignal>
 
 #ifdef USE_SIGNALFD
 #include <pthread.h>
 #endif
 
-#include <signal.h>
-
 class SignalMonitor final : private SocketMonitor {
 #ifdef USE_SIGNALFD
 	SignalFD fd;