[fileio] fix a clang warning about default ctor + const member

It seems the strict c++ spec used to state that a const class could only
be constructed if it had an explicitly define constructor - even if there
was an "obvious" default constructor, that wouldn't leave uninitialized
items.

A defect was filed, and later accepted:

http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253

...but it seems clang, as of Apple clang version 11.0.0, still warns if
you're using --std=c++11

(GCC lets this pass.)

lib/fileio/primUpdater.h

@@ -37,7 +37,11 @@ class UsdMayaPrimUpdater
     UsdMayaPrimUpdater(const MFnDependencyNode& depNodeFn,
                        const SdfPath& usdPath);
 
-    UsdMayaPrimUpdater() = default;
+    // clang errors if you use "= default" here, due to const SdfPath member
+    //    see: http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253
+    // ...which it seems clang only implements if using newer version + cpp std
+    UsdMayaPrimUpdater() {}
+
     virtual ~UsdMayaPrimUpdater() = default;
 
     enum class Supports {