Remove a dead header (<netutils/dhcp.h>).

9c07eb9dd83a4b62a24836cc82a0ab5e7ae08f7d removed the implementation, but
missed the header file.

Bug: http://b/27192644
Change-Id: I809a3eb0081b05783b70dbca0274cdde6b1b31a6
diff --git a/libnetutils/dhcptool.c b/libnetutils/dhcptool.c
index a2d3869..d23afd3 100644
--- a/libnetutils/dhcptool.c
+++ b/libnetutils/dhcptool.c
@@ -20,9 +20,10 @@
 #include <stdbool.h>
 #include <stdlib.h>
 
-#include <netutils/dhcp.h>
 #include <netutils/ifc.h>
 
+extern int do_dhcp(char*);
+
 int main(int argc, char* argv[]) {
   if (argc != 2) {
     error(EXIT_FAILURE, 0, "usage: %s INTERFACE", argv[0]);