Fixes compiler error (AppleClang,Gcc) in using std::atomic values as printf arguments (#826)

Compiler error in AppleClang (Xcode8) is

/Users/mark/dev/Thirdparty/OSL-1.9/src/liboslcomp/ast.cpp:76:33: error: call to implicitly-deleted copy constructor of 'std::atomic<int>'
                             i, node_counts[i], node_counts_peak[i]);
                                ^~~~~~~~~~~~~~

Compiler error in Gcc (4.8) is

/home/mark/dev/Thirdparty/OSL-1.9/src/liboslcomp/ast.cpp:76:68: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
                              i, node_counts[i], node_counts_peak[i]);
                                                                    ^
Believe this is a problem because printf accepts arguments by value, whereas OIIO 1.8's Strutil::printf takes by reference, and you can't copy a std::atomic.

Author: markfinal

src/liboslcomp/ast.cpp

@@ -73,7 +73,7 @@ ScopeExit print_node_counts ([](){
                              i, node_counts[i], node_counts_peak[i]);
 #else
             printf("ASTNode type %2d: %5d   (peak %5d)\n",
-                             i, node_counts[i], node_counts_peak[i]);
+                             i, node_counts[i].load(), node_counts_peak[i].load());
 #endif
 });
 }