[PATCH] fix build of targets not implementing add_stmt_cost

[Alexander Monakov]

On Tue, 5 May 2020, Richard Biener wrote:

> 
> Pushed as obvious.
> 
> C++ makes mismatched prototype and implementation OK.

(because of overloads)

I think this would have been caught if GCC enabled -Wmissing-declarations
during bootstrap, and the main reason we have this problem is that the
similar warning in C is named differently (-Wmissing-prototypes), and
we didn't change -Wmissing-prototypes to -Wmissing-declarations when
transitioning to C++ even though the former is a complete no-op for C++.

This is PR bootstrap/91972.

Alexander