Fix Python 2.4 build break

This fixes a build failure against Python 2.4 by casting away "const"
on the second argument to PyObject_GetAttrString.  Similar casts to
support Python 2.4 were already present in a number of other places.

gdb/
2014-03-16  Ulrich Weigand  <uweigand@de.ibm.com>

	* python/py-value.c (get_field_flag): Cast flag_name argument to
	PyObject_GetAttrString to support Python 2.4.