libldb: Fix const char in pyldb

libldb: Fix const char in pyldb

../lib/ldb/pyldb.c: In function ‘PyDict_AsMessage’:
../lib/ldb/pyldb.c:90:22: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
 #define PyStr_AsUTF8 PyUnicode_AsUTF8
                      ^~~~~~~~~~~~~~~~
../lib/ldb/pyldb.c:1359:19: note: in expansion of macro ‘PyStr_AsUTF8’
   char *key_str = PyStr_AsUTF8(key);
                   ^~~~~~~~~~~~
../lib/ldb/pyldb.c: In function ‘py_ldb_msg_getitem_helper’:
../lib/ldb/pyldb.c:3336:7: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  name = PyStr_AsUTF8(py_name);
       ^
../lib/ldb/pyldb.c: In function ‘py_ldb_msg_setitem’:
../lib/ldb/pyldb.c:3502:12: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  attr_name = PyStr_AsUTF8(name);
            ^

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Uri Simchoni <uri@samba.org>