wmcliphist: Add <sys/stat.h> to includes.

Otherwise, we get the following compiler warning:
history.c:289:2: warning: implicit declaration of function ‘chmod’
[-Wimplicit-function-declaration]
  if ((chmod(fname, S_IRUSR|S_IWUSR)) != 0) {
  ^

wmcliphist/wmcliphist.h

@@ -11,6 +11,7 @@
 #include <unistd.h>
 #include <ctype.h>
 #include <errno.h>
+#include <sys/stat.h>
 
 #include <fcntl.h>
 #include <regex.h>