Update dptr.h

DPTR_DECLARE_PUBLIC宏中代码应该有误, 应该是
#define DPTR_DECLARE_PUBLIC(Class) \
    inline Class& q_func() { return *static_cast<Class*>(dptr_ptr()); } \
    inline const Class& q_func() const { return *static_cast<const Class*>(dptr_ptr()); } \
    friend class Class;
Merge pull request #1 from slug404/patch-1

Update dptr.h