Fixed "could not connect to display" error (#771)

This is a pertaining issue in Linux systems where the DISPLAY variable isn't forwarded. The error which is fixed is; "QXcbConnection: Could not connect to display". Users before have raised issues on this [here](https://github.com/lifting-bits/mcsema/issues?q=QXcbConnection%3A+Could+not+connect+to+display)

Author: burntfalafel

tools/mcsema_disass/ida7/disass.py

@@ -42,6 +42,7 @@ def execute(args, command_args):
  env["HOME"] = os.path.expanduser('~')
  env["IDA_PATH"] = os.path.dirname(args.disassembler)
  env["PYTHONPATH"] = os.path.dirname(ida_dir)
+ env["DISPLAY"] = ":0"
  if "SystemRoot" in os.environ:
  env["SystemRoot"] = os.environ["SystemRoot"]