Include ax.h in ax-gdb.h

Include ax.h in ax-gdb.h

$ make check-headers CHECK_HEADERS="ax-gdb.h"
...

../../binutils-gdb/gdb/ax-gdb.h:104:8: error: 'agent_expr_up' does not name a type
 extern agent_expr_up gen_trace_for_expr (CORE_ADDR, struct expression *,
        ^

gdb:

2017-01-06  Yao Qi  <yao.qi@linaro.org>

	* ax-gdb.h: Include "ax.h"