BaseTools: Fix string concatenation

BaseTools: Fix string concatenation

Using Python 3.7.2 on win32, when printing a FileBuildRule
instance, the following error occurs:
File "edk2\BaseTools\Source\Python\AutoGen\BuildEngine.py",
line 177, in __str__
  DestString = ", ".join(self.DestFileList)
  TypeError: sequence item 0: expected str instance, PathClass found

This patch converts each PathClass element of the list to a string
instance before concatenating them.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>