Add missing header <sys/stat.h> to device_manager.cc

device_manager.cc needs the definition for 'struct stat' from this
header.  Currently, it relies on getting it included indirectly through
<libudev.h>.

The dependency on <sys/stat.h> was unused in <libudev.h> and was removed
upstream on commit 2eec67acbb0059 (first included in systemd/udev v220.)
Upgrade to that or a later version would cause the mtpd build to break.

Make the dependency explicit here, to prevent any future breakage when
the package owning <libudev.h> is upgraded.

BUG=None
TEST=Built packages on swanky systemd v226.
  Confirmed no build breakage with current versions.

Change-Id: I46e55e21a5206d605a2244ff93486b4c6bdf7aaa
Reviewed-on: https://chromium-review.googlesource.com/325760
Commit-Ready: Filipe Brandenburger <filbranden@chromium.org>
Tested-by: Filipe Brandenburger <filbranden@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/device_manager.cc b/device_manager.cc
index da8d5f5..d040428 100644
--- a/device_manager.cc
+++ b/device_manager.cc
@@ -5,6 +5,7 @@
 #include "device_manager.h"
 
 #include <libudev.h>
+#include <sys/stat.h>
 
 #include <set>