Explicitly use CreateFileA in readlink

Summary:
In Unicode enabled projects, this errors because CreateFile aliases
CreateFileW, which takes wchar_t* not char*.

Reviewed By: Orvid, yfeldblum

Differential Revision: D4878424

fbshipit-source-id: b44b369c0533e74163f68d95c2bf353584033731

folly/portability/Unistd.cpp

@@ -205,7 +205,8 @@ ssize_t readlink(const char* path, char* buf, size_t buflen) {
     return -1;
   }
 
-  HANDLE h = CreateFile(path,
+  HANDLE h = CreateFileA(
+      path,
       GENERIC_READ,
       FILE_SHARE_READ,
       nullptr,