Update git submodules

* Update external/libffi from branch 'master'
  to 56b44d2c902ddef15ec2054bbbdab8bbeefdf7a8
  - Merge "Remove unnecessary win32.S source file."
  - Remove unnecessary win32.S source file.
    
    Bug: http://b/162610242
    Test: mmma external/libffi
    Change-Id: I9f1f980af1545a0de8f7897291b1c5206bf17b02
    
1 file changed