Findutf8cpp.cmake: add utf8cpp include subdir

According to upstream, it installs its headers in a subdir, see:
https://github.com/nemtrif/utfcpp/blob/master/CMakeLists.txt#L23

We could also include the subdir in the sources,
but it'd mean not being compatible with previous versions without the subdir:
see https://packages.debian.org/sid/amd64/libutfcpp-dev/filelist

CMake/Findutf8cpp.cmake

 find_path(utf8cpp_INCLUDE_DIR
   NAMES utf8.h
+  PATH_SUFFIXES utf8cpp
   DOC "utf8cpp include directory")
 mark_as_advanced(utf8cpp_INCLUDE_DIR)