Fix build with gcc-10

Fix build with gcc-10

From an e-mail from Jeff Law <law@redhat.com>:

Subject: Minor problem in pdns, dnsdist and pdns-recursor packages in Fedora

[ All three packages have embedded copies of the same problematic code
and the same patch fixes all three. ]

Red Hat's compiler team continues to try and be proactive in identifying
issues that will arise as a result of the introduction of a new GCC
release into Fedora each spring.

You're being contacted because a package you maintain in Fedora is going
to fail to build with gcc-10 in the spring.  Yes, I know that's a few
months away, but it's far easier to fix this stuff proactively now than
wait.

Fixing it now also means that your package will continue to be built
with testing versions of gcc-10 as we proceed through the development
process thus allowing additional issues to be caught early.

Your particular package will fail due to an uninstantiated template for
AsyncLoader<Request>.  These kinds of problems are relatively common due to
changes in the tuning of the inliner for gcc-10:

> BUILDSTDERR: /usr/bin/ld: webserver.o: in function `WebServer::serveConnection(std::shared_ptr<Socket>) const':
> BUILDSTDERR: /builddir/build/BUILD/pdns-4.2.1/pdns/webserver.cc:373: undefined reference to `YaHTTP::AsyncLoader<YaHTTP::Request>::feed(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
> BUILDSTDERR: collect2: error: ld returned 1 exit status
> BUILDSTDERR: make[3]: *** [Makefile:2751: ixfrdist] Error 1
>

The attached patch arranges for an instance to be instantiated when
compiling reqresp.cpp and is sufficient to fix this problem.  The
choice of reqresp.cpp fairly arbitrary IIRC.

Ideally you'll with upstream to get this fixed, but a Fedora patch is
clearly OK as well.   I'll install the attached fix into Fedora in a
week or so if I haven't heard from you.

Jeff

(cherry picked from commit ffb885e937c27bb1c62dee8f18c58ae0d5d52d39)