tests/extmod/vfs_rom.py: Import errno for test.

It's needed by the test.  This previously passed because the compiler
(actually parser) optimises away errno constants.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/vfs_rom.py

@@ -1,7 +1,7 @@
 # Test VfsRom filesystem.
 
 try:
-    import sys, struct, os, uctypes, vfs
+    import errno, sys, struct, os, uctypes, vfs
 
     vfs.VfsRom
 except (ImportError, AttributeError):