fix -b as only options when calling msfvenom

msfvenom

@@ -135,6 +135,7 @@ def parse_args(args)
   end
 
   opt.on('-b', '--bad-chars     <list>', String, 'The list of characters to avoid example: \'\x00\xff\'') do |b|
+    require 'rex/text'
     opts[:badchars] = Rex::Text.hex_to_raw(b)
   end