Add pybind11 dependency.

Tracking 'stable' branch.
diff --git a/.gitmodules b/.gitmodules
index 6d7ec53..a60c692 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -38,3 +38,7 @@
 [submodule "third_party/spirv_headers"]
 	path = third_party/spirv_headers
 	url = https://github.com/KhronosGroup/SPIRV-Headers.git
+[submodule "third_party/pybind11"]
+	path = third_party/pybind11
+	url = https://github.com/pybind/pybind11.git
+	branch = stable
diff --git a/third_party/pybind11 b/third_party/pybind11
new file mode 160000
index 0000000..80d4524
--- /dev/null
+++ b/third_party/pybind11
@@ -0,0 +1 @@
+Subproject commit 80d452484c5409444b0ec19383faa84bb7a4d351