Fix implicit functions declarations

--- a/xbatt.c
+++ b/xbatt.c
@@ -47,6 +47,9 @@
 #include <time.h>
 #include <sys/file.h>
 #include <sys/ioctl.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <X11/IntrinsicP.h>
 #include <X11/StringDefs.h>
 #include <X11/Intrinsic.h>
 #include <X11/Shell.h>