sfst build fails via pip

[bluebottlewize]

CMake Error at CMakeLists.txt:21 (add_subdirectory):
        add_subdirectory given source "pybind11" which is not an existing
        directory.
      
      
      CMake Error at python/CMakeLists.txt:16 (pybind11_add_module):
        Unknown CMake command "pybind11_add_module".

Seems to be because the sub module pybind11 is not being cloned. When i cloned it, the build process succeeds.

Due to this, the mlmorph tool installation also fails via pip.