add namespace std for sort function. (#453)
add namespace std for sort function. (#453)

Compiling in some environments fails due to name ambiguous error.
sppalkia/weld,sppalkia/weld,weld-project/weld,sppalkia/weld,weld-project/weld,weld-project/weld,sppalkia/weld,weld-project/weld,sppalkia/weld,weld-project/weld