[gdb/testsuite] Change kfail into xfail in gdb.ada/packed_tagged.exp

[gdb/testsuite] Change kfail into xfail in gdb.ada/packed_tagged.exp

Test-case gdb.ada/packed_tagged.exp contains a kfail:
...
setup_kfail "gnat compiler bug" *-*-*
...

Kfails are used to indicate problems in gdb, xfails are used to indicate
problems in the environment.

A bug in the gnat compiler is a problem in the environment rather than gdb.

Fix this by changing the kfail into an xfail.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-11  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/packed_tagged.exp: Change kfail into xfail.