Don't try to use the lwip library on Darwin. (#36258)
Don't try to build against the library, and don't check out the submodule for
the darwin platform.
diff --git a/.gitmodules b/.gitmodules
index 824bd99..4eca51b 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -332,6 +332,7 @@
[submodule "third_party/lwip/repo"]
path = third_party/lwip/repo
url = https://github.com/lwip-tcpip/lwip.git
+ excluded-platforms = darwin
[submodule "third_party/abseil-cpp/src"]
path = third_party/abseil-cpp/src
url = https://github.com/abseil/abseil-cpp.git
