Add -Wl,-z,undefs to linux links (#90)

If the user's toolchain / global settings imply -z,defs, this is
necessary similar to the macOS flags to allow libpython symbols to be
undefined

build_defs.bzl

diff --git a/build_defs.bzl b/build_defs.bzl
index 5c88f1c..1b10d11 100644
--- a/build_defs.bzl
+++ b/build_defs.bzl
@@ -68,7 +68,10 @@
         linkopts = linkopts + select({
             "@platforms//os:osx": ["-undefined", "dynamic_lookup"],
             Label("@pybind11//:msvc_compiler"): [],
-            "//conditions:default": ["-Wl,-Bsymbolic"],
+            "//conditions:default": [
+                "-Wl,-Bsymbolic",
+                "-Wl,-z,undefs",
+            ],
         }),
         linkshared = 1,
         tags = tags,