An upstream change to Clang will require that size_t be explicitly

brought in from a header, rather than implicitly used, as it allows
delineation between different declarations of size_t.
This change includes cstddef explicitly in gf.

Closes PixarAnimationStudios#3535

(Internal change: 2359159)

Author: dgovil

pxr/base/gf/half.h

@@ -18,16 +18,18 @@
 #include "pxr/base/gf/ilmbase_halfLimits.h"
 #include "pxr/base/gf/traits.h"
 
+#include <cstddef>
+
 PXR_NAMESPACE_OPEN_SCOPE
 
 /// A 16-bit floating point data type.
 using GfHalf = pxr_half::half;
 
 namespace pxr_half {
     /// Overload hash_value for half.
-    inline size_t hash_value(const half h) { return h.bits(); }
+    inline std::size_t hash_value(const half h) { return h.bits(); }
     // Explicitly delete hashing via implicit conversion of half to float
-    size_t hash_value(float) = delete;
+    std::size_t hash_value(float) = delete;
 }
 
 template <>