gdb: move annotation_level declaration/definition to annotate.{h,c}

The declaration of annotation_level is currently in defs.h, while the
definition is in stack.c.  I don't really understand why that variable
would live in stack.c, it seems completely unrelated.  Move it to
annotate.c, and move the declaration to annotate.h.

Change-Id: I6cf8e9bd20e83959bdf5ad58dd008b6e1187d7d8
Approved-By: Tom Tromey <tom@tromey.com>

gdb/annotate.c

@@ -29,6 +29,10 @@
 #include "source-cache.h"
 #include "ui.h"
 
+/* See annotate.h.  */
+
+int annotation_level = 0;
+
 /* Prototypes for local functions.  */
 
 static void print_value_flags (struct type *);