[contrib][pzstd] Fix build issue with gcc-5

gcc-5 didn't like the l-value overload for defaulted operator=. There is
no reason it needs to be l-value overloaded, so just remove it.

I'm not sure why the build broke for @mckaygerhard in Issue #2811, since
this code hasn't changed since it was added. But, there is no harm in
fixing it.

Fixes issue #2811.

contrib/pzstd/utils/Buffer.h

@@ -45,7 +45,7 @@ class Buffer {
       : buffer_(buffer), range_(data) {}
 
   Buffer(Buffer&&) = default;
-  Buffer& operator=(Buffer&&) & = default;
+  Buffer& operator=(Buffer&&) = default;
 
   /**
    * Splits the data into two pieces: [begin, begin + n), [begin + n, end).