pybind11 is not found correctly when using python venv

[fifield]

https://github.com/Xilinx/XRT/blob/5d6c9134aecdb88fa171862603a8c25fcad85e40/src/python/pybind11/CMakeLists.txt#L40-L41

This code assumes that if building on ubuntu then packages are in the directory dist-packages. This is not true if the user (me) is using venv, which puts things in site-packages instead. This is problematic because use of venv is common and is the standard way to create python environments. My current workaround is to hardcode correct search path in to the find_package line.