ntp_tiles: Include <stddef.h> for size_t

IWYU. size_t needs to come somewhere, so rather than indirectly including it
via <utility>, explicitly include <stddef.h>.

This fixes the libstdc++ build after commit 01ca125f ("[NTP] Define
constants for max number of Most Visited and Custom Links"):

In file included from ../../components/ntp_tiles/constants.cc:5:
../../components/ntp_tiles/constants.h:15:14: error: unknown type name 'size_t'; did you mean 'std::size_t'?
extern const size_t kMaxNumCustomLinks;
             ^~~~~~
             std::size_t
../../../../../../usr/bin/../lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8/x86_64-redhat-linux/bits/c++config.h:2182:26: note: 'std::size_t' declared here
  typedef __SIZE_TYPE__         size_t;
                                ^
../../components/ntp_tiles/constants.cc:9:7: error: unknown type name 'size_t'; did you mean 'std::size_t'?
const size_t kMaxNumCustomLinks = 10;
      ^~~~~~
      std::size_t
../../../../../../usr/bin/../lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8/x86_64-redhat-linux/bits/c++config.h:2182:26: note: 'std::size_t' declared here
  typedef __SIZE_TYPE__         size_t;
                                ^

Bug: 957519
Change-Id: I347bef878b954779442a640213952432b329411c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1608744
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659113}

components/ntp_tiles/constants.h

@@ -5,6 +5,8 @@
 #ifndef COMPONENTS_NTP_TILES_CONSTANTS_H_
 #define COMPONENTS_NTP_TILES_CONSTANTS_H_
 
+#include <stddef.h>
+
 #include <utility>
 
 namespace ntp_tiles {