hb-private.hh: Add fallback implementation for round()

For pre-C99 compilers that do not support round(), we need to have a
simplistic implementation for it, when it is not detected during build
configuration, either via CMake or autotools, by using floor() and
ceil(), which are provided in the pre-C99 compilers.

Please see discussion at commit 86a0ac2 for more details for re-adding
this patch.