shill: cellular: Update ModemManager header inclusion.

The upstream ModemManager has moved public header files from <prefix>/mm
to <prefix>/ModemManager. This CL updates the ModemManager header
inclusion in shill.

BUG=chromium-os:32260
TEST=Build and run unit tests.

Change-Id: I86d0e53ad24f35e7b14385411533a7a0e3e4ed87
Reviewed-on: https://gerrit.chromium.org/gerrit/26450
Reviewed-by: Gary Morain <gmorain@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
diff --git a/modem_manager.cc b/modem_manager.cc
index 155099f..4c06609 100644
--- a/modem_manager.cc
+++ b/modem_manager.cc
@@ -10,7 +10,7 @@
 #include <base/logging.h>
 #include <base/stl_util.h>
 #include <mm/mm-modem.h>
-#include <mm/ModemManager-names.h>
+#include <ModemManager/ModemManager-names.h>
 
 #include "shill/error.h"
 #include "shill/modem.h"