scripts/fsrefs: Fix -v

For this script verbose mode was added in 57bca1f0 (Add -v option to
control whether tracebacks are printed.) and was later broken in
6dd809d2 (Converted to a buildout and converted externals to
dependencies.) - the change forgot to add global specifier for VERBOSE
variable while moving its assignment into function context.

As a result currently we have

    $ python ./src/ZODB/scripts/fsrefs.py -v data.fs
    Traceback (most recent call last):
      File "./src/ZODB/scripts/fsrefs.py", line 155, in <module>
        main()
      File "./src/ZODB/scripts/fsrefs.py", line 104, in main
        VERBOSE += 1
    UnboundLocalError: local variable 'VERBOSE' referenced before assignment

Fix it.

src/ZODB/scripts/fsrefs.py

@@ -94,6 +94,7 @@ def report(oid, data, serial, missing):
     print()
 
 def main(path=None):
+    global VERBOSE
     if path is None:
         import sys
         import getopt