In wrappers, guard against #including dirs.

The wrappers tools try to read all #included headers in order to get all
the preprocessor macros that they might need to properly parse the code.
However, if a header cannot be loaded, they are supposed to be ignored.
David DeMarle recently discovered that if the #include directive found
a directory with the name of the header that it was looking for, then the
wrapper tools would fail.  I've added an "isdir" check to guard against
this.

Change-Id: I4847b8806b3e28a42aa299dce0dfe58254c20084