Fix SciPy pybind11 build dependency

[lazycat11]

Summary

• add SciPy's pybind11 build dependency to hostpython_prerequisites

Rationale

SciPy 1.16.2 declares pybind11>=2.13.2,<3.1.0 in pyproject.toml and uses dependency('pybind11', version: '>=2.13.2') in its Meson build. The recipe already installs NumPy and Cython as host Python prerequisites, but pybind11 was missing, so Meson could fail to resolve the pybind11 dependency during the isolated wheel build.

This keeps the fix local to the SciPy recipe and avoids changing the shared MesonRecipe flow.

Validation

• python3 -m py_compile pythonforandroid/recipes/scipy/__init__.py pythonforandroid/recipe.py
• git diff --check