Minor, make CreateFile compatible with the disabled writable flag

"writable" is not in use and probably we can go in the reverse direction
and remove the flags that are useful for enabled writable, this is just
for the sake of completeness however.

src/hb-blob.cc

@@ -556,8 +556,9 @@ hb_blob_create_from_file (const char *file_name)
   if (unlikely (file->contents == MAP_FAILED)) goto fail;
 
 #elif defined(_WIN32) || defined(__CYGWIN__)
-  HANDLE fd = CreateFile (file_name, GENERIC_READ, FILE_SHARE_READ, nullptr,
-			  OPEN_EXISTING,
+  HANDLE fd = CreateFile (file_name,
+			  writable ? GENERIC_READ|GENERIC_WRITE : GENERIC_READ,
+			  FILE_SHARE_READ, nullptr, OPEN_EXISTING,
 			  FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, nullptr);
 # define CLOSE CloseHandle