docs: mention that python package packaging is required on python 3.12+

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26857>

docs/install.rst

@@ -36,6 +36,8 @@ Third party/extra tools.
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 - `Python <https://www.python.org/>`__ - Python 3.6 or newer is required.
+- Python package ``packaging`` is required on Python 3.12+:
+  ``pip install packaging``
 - `Python Mako module <https://www.makotemplates.org/>`__ - Python Mako
   module is required. Version 0.8.0 or later should work.
 - Lex / Yacc - for building the Mesa IR and GLSL compiler.

docs/meson.rst

@@ -66,7 +66,7 @@ Then install Meson using pip
 
 .. code-block:: console
 
-   py -3 -m pip install meson mako
+   py -3 -m pip install meson packaging mako
 
 You may need to add the Python 3 scripts directory to your path for
 Meson.