fix(vllm): seed pybind11 for fastsafetensors build under --no-build-isolation

fastsafetensors==0.3 (transitive dep of vllm) imports pybind11 in
setup.py without declaring it in build-system.requires. With
--no-build-isolation it has to already exist in the venv, otherwise the
wheel build fails with ModuleNotFoundError on arm64 L4T CUDA 13 (and
any other profile that picks up vllm 0.20.0).

Assisted-by: Claude:claude-opus-4-7 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/python/vllm/requirements-install.txt

@@ -1,6 +1,11 @@
 # mabma does not specify it's build dependencies per PEP517, so we need to disable build isolation
 # this also means that we need to install the basic build dependencies into the venv ourselves
 # https://github.com/Dao-AILab/causal-conv1d/issues/24
+#
+# pybind11 is needed to build fastsafetensors==0.3 (transitive dep of vllm) — its
+# setup.py imports pybind11 unconditionally but doesn't declare it in
+# build-system.requires, so under --no-build-isolation we have to seed it ourselves.
 packaging
 setuptools
-wheel
+wheel
+pybind11