Add empty virtual destructor to TraceResolverImplBase base class

[christophebedard]

Fixes some warnings:

... bombela/backward-cpp/backward.hpp:1210:7: warning: ‘class backward::TraceResolverImplBase’ has virtual functions and accessible non-virtual destructor [-Wnon-virtual-dtor]
 1210 | class TraceResolverImplBase {
      |       ^~~~~~~~~~~~~~~~~~~~~
... bombela/backward-cpp/backward.hpp:1243:7: warning: base class ‘class backward::TraceResolverImplBase’ has accessible non-virtual destructor [-Wnon-virtual-dtor]
 1243 | class TraceResolverLinuxBase : public TraceResolverImplBase {
      |       ^~~~~~~~~~~~~~~~~~~~~~

I considered using a = default virtual destructor, but I wasn't sure about backwards compatibility.

[bombela]

Yeah the code should be compatible with C++98.