magfit.py: add DCM as attitude choice

Author: peterbarker

Diff for: MAVProxy/modules/lib/magfit.py

@@ -579,6 +579,8 @@ def init_ui(self):
             att_choices.append('XKF1')
         if self.have_msg('XKY0'):
             att_choices.append('XKY0')
+        if self.have_msg('DCM'):
+            att_choices.append('DCM')
 
         orientation_choices = [ r.name for r in rotations ]