[draco] Fix build failure on gcc 11

[JackBoosY]

Fix build failure:

[snip]
In file included from hash_utils.cc:15:
draco/core/hash_utils.h:27:1: error: 'size_t' does not name a type
   27 | size_t HashCombine(T1 a, T2 b) {
      | ^~~~~~
draco/core/hash_utils.h:21:1: note: 'size_t' is defined in header '<cstddef>'; did you forget to '#include <cstddef>'?
   20 | #include <functional>
  +++ |+#include <cstddef>
   21 | 
draco/core/hash_utils.h:34:1: error: 'size_t' does not name a type
   34 | size_t HashCombine(T a, size_t hash) {
      | ^~~~~~
draco/core/hash_utils.h:34:1: note: 'size_t' is defined in header '<cstddef>'; did you forget to '#include <cstddef>'?
draco/core/hash_utils.h:44:43: error: 'size_t' has not been declared
   44 | uint64_t FingerprintString(const char *s, size_t len);
      |                                           ^~~~~~
draco/core/hash_utils.h:49:3: error: 'size_t' does not name a type
   49 |   size_t operator()(const T &a) const {
      |   ^~~~~~
draco/core/hash_utils.h:49:3: note: 'size_t' is defined in header '<cstddef>'; did you forget to '#include <cstddef>'?
draco/core/hash_utils.h:58:3: error: 'size_t' does not name a type
   58 |   size_t ValueHash(const V &val) const {
      |   ^~~~~~
draco/core/hash_utils.h:58:3: note: 'size_t' is defined in header '<cstddef>'; did you forget to '#include <cstddef>'?
[snap]

Use the official commit google/draco#636.

Fixes #17963.

[NancyLi1013]

/azp run

[azure-pipelines]

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