FindGraphviz: Fix finding graphviz not installed in system directories

[traversaro]

When graphviz is not installed in system directories, its headers are installed in a path like:

<install_prefix>/include/graphviz/cgraph.h

Before this PR, even if <install_prefix> is added to the CMAKE_PREFIX_PATH, the snippet of code:

find_path(Graphviz_INCLUDE_DIR
          NAMES cgraph.h
          PATHS ${Graphviz_POSSIBLE_INCDIRS}
                /usr/include
                /usr/include/graphviz
                /usr/local/include
                /usr/local/include/graphviz
          HINTS ${Graphviz_PKG_INCLUDE_DIR})

would not find it, and would mark Graphviz as not found. In this PR, we add the PATH_SUFFIXES option so that regardless of the installation prefix Graphvix is correctly found. This for example would fix finding Graphviz when it is installed in a conda environment.

While this file is in 3rdparty/qgv, I opened directly a PR here as the original from which the file was obtained is now archived: https://github.com/nbergont/qgv/ .

[traversaro]

Thanks @S-Dafarra @diegoferigo !