Wrap PyObject_Get/HasAttrString in a function with second arg having const qualifier.

Wrap PyObject_Get/HasAttrString in a function with second arg having const qualifier.

This is done to avoid errors when compiled with -Werror against Python-2.4
which did not have the const qualifier for the second argument of these
functions.

gdb/
	* python/python-internal.h (gdb_PyObject_GetAttrString)
	(gdb_PyObject_HasAttrString): New inline function definitions.
	* py-value.c (get_field_flag): Remove the now unnecessary cast to
	char * of the second argument to PyObject_GetAttrString.