Fix ODR warning in observable.h

Fix ODR warning in observable.h

observable.h triggers an ODR warning because this line:

    extern observable<struct target_ops */* target */> target_changed;

... may be the only declaration of "struct target_ops" in scope
(depending on the particular .c file) -- and this declares it in a
namespace, resulting in confusion.

This patch fixes the problem by adding a forward declaration.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=22395