gdb: add includes in target-debug.h

Editing target-debug.h with clangd shows a bunch of errors.  Add some
includes to fix that (make target-debug.h include what it uses).

Change-Id: I49075a171e6875fa516d6b2ce56b4a03ac7b3376
diff --git a/gdb/target-debug.h b/gdb/target-debug.h
index 20ba1f7..7f1b6d0 100644
--- a/gdb/target-debug.h
+++ b/gdb/target-debug.h
@@ -38,6 +38,10 @@
    more simply done there, and target_xfer_partial additionally
    bypasses the debug target.  */
 
+#include "gdbsupport/x86-xstate.h"
+#include "target.h"
+#include "target/wait.h"
+#include "target/waitstatus.h"
 
 /* Helper macro.  */