Include a header required for kill()

Yet, this program remains fairly unportable.

sdn.1

@@ -67,8 +67,8 @@ The viewer program to be launched by the F3 key binding as well as to show
 the internal help message.  If none is set, it defaults to
 .Xr less 1 .
 .It Ev VISUAL , Ev EDITOR
-The editor program to be launched by the F4 key binding.  If none is set,
-it defaults to
+The editor program to be launched by the F4 key binding.  If neither variable
+is set, it defaults to
 .Xr vi 1 .
 .El
 .Sh FILES

sdn.cpp

@@ -42,6 +42,7 @@
 #include <grp.h>
 #include <libgen.h>
 #include <time.h>
+#include <signal.h>
 
 #include <sys/inotify.h>
 #include <sys/xattr.h>