Add missing guard to prevent adding code to a Windows build

Author: tblah

flang/include/flang/Runtime/extensions.h

@@ -45,8 +45,11 @@ std::int64_t RTNAME(Signal)(std::int64_t number, void (*handler)(int));
 void RTNAME(Sleep)(std::int64_t seconds);
 
 // GNU extension function ACCESS(NAME, MODE)
+// TODO: not supported on Windows
+#ifndef _WIN32
 std::int64_t FORTRAN_PROCEDURE_NAME(access)(const char *name,
     std::int64_t nameLength, const char *mode, std::int64_t modeLength);
+#endif
 
 } // extern "C"
 #endif // FORTRAN_RUNTIME_EXTENSIONS_H_