quiche.h: ensure size_t and ssize_t are defined
quiche.h: ensure size_t and ssize_t are defined

Ensure that check_symbol_exists calls in CMake do not fail due to
missing headers. The minimal reproducer can be stripped down to:

    echo '#include <quiche.h>' | gcc -Iinclude -fsyntax-only -xc -

A similar (untested) fix for Windows was taken from
https://github.com/cloudflare/quiche/issues/450

cloudflare/quiche,cloudflare/quiche,cloudflare/quiche