binman: Init align_default in entry_Section

This should be set up in the init function, to avoid a warning about a
property not set up there. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/etype/section.py

@@ -168,6 +168,7 @@ class Entry_section(Entry):
         self._end_4gb = False
         self._ignore_missing = False
         self._filename = None
+        self.align_default = 0
 
     def IsSpecialSubnode(self, node):
         """Check if a node is a special one used by the section itself