Fixing problem with the lib/include/avx512vlintrin.h file. 
Adding one more _ to the prefix of _extension__ -> __extension__.

Differential Revision: http://reviews.llvm.org/D16985


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261518 91177308-0d34-0410-b5e6-96231b3b80d8