shill: Fix header guards to conform to coding style.

BUG=chromium:385849
TEST=Tested the following:
1. `USE='cellular wimax' FEATURES=test emerge-$BOARD platform2`
2. `cpplint.py --filter=-build/include_order shill/*.h` reports no
   'build/header_guard' issue.

Change-Id: I1a3972a1f88e5001550eccd561633bd4e5c5faba
Reviewed-on: https://chromium-review.googlesource.com/206680
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/dhcp_proxy_interface.h b/dhcp_proxy_interface.h
index 239095e..35636cf 100644
--- a/dhcp_proxy_interface.h
+++ b/dhcp_proxy_interface.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef SHILL_DHCP_PROXY_INTERFACE_
-#define SHILL_DHCP_PROXY_INTERFACE_
+#ifndef SHILL_DHCP_PROXY_INTERFACE_H_
+#define SHILL_DHCP_PROXY_INTERFACE_H_
 
 #include <string>
 
@@ -21,4 +21,4 @@
 
 }  // namespace shill
 
-#endif  // SHILL_DHCP_PROXY_INTERFACE_
+#endif  // SHILL_DHCP_PROXY_INTERFACE_H_