Drop a REQUIRES: lldb on a dexter regression test

As this is a test that actually gets to operating the debugger, it
needs to be limited to scenarios where the debugger is available.

(We'll file this in the set of things Dexter doesn't handle gracefully..)

debuginfo-tests/dexter/feature_tests/subtools/test/label_another_line.cpp

@@ -1,3 +1,6 @@
+// REQUIRES: lldb
+// UNSUPPORTED: system-windows
+//
 // Purpose:
 //    Check that the optional keyword argument 'on_line' makes a \DexLabel label
 //    that line instead of the line the command is found on.