I found this issue when I was debugging something else on IA-64.  Both
ax-gdb.h and parser-defs.h could be made more self-contained by forward
declaring types or including the necessary header files.  This commit does
this.

2013-06-26  Sergio Durigan Junior  <sergiodj@redhat.com>

	* ax-gdb.h (union exp_element): Forward declare.
	* parser-defs.h: Include expression.h.