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

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>