Declare __lzcnt* as Windows built-in

This avoids warnings about missing declarations.

Author: tautschnig

src/ansi-c/windows_builtin_headers.h

@@ -1 +1,4 @@
 int __assume(int);
+unsigned short __lzcnt16(unsigned short value);
+unsigned int __lzcnt(unsigned int value);
+unsigned __int64 __lzcnt64(unsigned __int64 value);