Avoid complaint warning on mingw

The mingw build currently issues a warning:

./../../src/gdb/utils.h:378:56: warning: ignoring attributes on template argument 'void(const char*, va_list)' {aka 'void(const char*, char*)'} [-Wignored-attributes]

This patch fixes the problem as suggested by Simon:

    https://sourceware.org/pipermail/gdb-patches/2024-April/207908.html



...that is, by changing the warning interceptor to a class with a
single 'warn' method.

Approved-By: Simon Marchi <simon.marchi@efficios.com>