[gdb/testsuite] Fix gdb.ada/mi_prot.exp

[gdb/testsuite] Fix gdb.ada/mi_prot.exp

When running test-case gdb.ada/mi_prot.exp with gcc 8.5.0, we run into:
...
(gdb) ^M
Expecting: ^(-stack-list-arguments --no-frame-filters 1[^M
]+)?(\^done,stack=.*[^M
]+[(]gdb[)] ^M
[ ]*)
-stack-list-arguments --no-frame-filters 1^M
^done,stack-args=[frame={level="0",args=[{name="<_object>",value="(ceiling_priority =\
> 97, local => 0)"},{name="v",value="5"},{name="<_objectO>",value="true"}]},frame={le\
vel="1",args=[{name="v",value="5"},{name="<_objectO>",value="true"}]},frame={level="2\
",args=[]}]^M
(gdb) ^M
FAIL: gdb.ada/mi_prot.exp: -stack-list-arguments --no-frame-filters 1 (unexpected out\
put)
...

Fix this by updating the regexp to expect "^done,stack-args=" instead of
"^done,stack=".

Tested on x86_64-linux.