gui/mp: Fix two strings not being translated in the UI

This is caused by a broken assumption on wmlxgettext's part. Because the
files did not have a textdomain directive anywhere, the translatable
strings in them were assigned to the wesnoth textdomain. The game engine
seems to have a different idea though, and includes them in an
unidentified textdomain that may or may not be wesnoth-lib, as per the
last #textdomain directive seen in the combined WML for the
data/gui/macros directory substitution.

This merits further examination of the preprocessor's code (and
wmlxgettext's logic), but for now this is a simple patch that fixes the
issue without breaking the string freeze or introducing any potential
regressions.

This fixes two particular strings mentioned in issue #2709:

 * "There are no custom options available for the selected era, game, or modification."
 * "Connected Players"

Vultraz made a similar fix in commit
8d1ec62 in master, but it breaks the
string freeze by virtue of moving the strings across textdomains.

[ci skip]

data/gui/macros/custom_options_tree.cfg

@@ -1,3 +1,4 @@
+#textdomain wesnoth
 #
 # Not a dialog, but a common portion of dialog layout used in at least two dialogs
 #

data/gui/macros/player_list_panel.cfg

@@ -1,3 +1,4 @@
+#textdomain wesnoth
 #
 # Common player list layout used in MP Staging and MP Join Game
 #