BUILD: tools: properly define ha_dump_backtrace() to avoid a build warning

BUILD: tools: properly define ha_dump_backtrace() to avoid a build warning

In resolve_sym_name() we declare a few symbols that we want to be able
to resolve. ha_dump_backtrace() was declared with a struct buffer instead
of a pointer to such a struct, which has no effect since we only want to
get the function's pointer, but produces a build warning with LTO, so
let's fix it.

This can be backported to 3.0.