Fix leading whitespace being ignored when searching for subtag start.

Reenables the check for "unknown xy referred to by id".

Fix for a bug report in the forum:

http://forums.wesnoth.org/viewtopic.php?p=519028#p519028

(It's not clear to me what the purpose of the subtag_depth variable is
though.)

data/tools/wmllint

@@ -1164,7 +1164,7 @@ def global_sanity_check(filename, lines):
         elif "[/multiplayer]" in lines[i]:
             in_multiplayer = False
         else:
-            if re.match(r"\[[a-z]", lines[i]):
+            if re.search(r"\[[a-z]", lines[i]):
                 subtag_depth += 1
             if "[/" in lines[i]:
                 subtag_depth -= 1