Update protobuf_deps.bzl to add a dependency on Abseil (#9752)

This is the most recent Abseil LTS branch, from November 2021. We do not
yet use Abseil for anything, but this change will make it possible to
start using it in Bazel when we're ready.

protobuf_deps.bzl

@@ -27,6 +27,15 @@ def protobuf_deps():
             ],
         )
 
+    if not native.existing_rule("com_google_absl"):
+        # Abseil LTS from November 2021
+        http_archive(
+          name = "com_google_absl",
+          sha256 = "b4e20d9e752a75c10636675691b1e9c2698e0764cb404987d0ffa77223041c19",
+          urls = ["https://github.com/abseil/abseil-cpp/archive/215105818dfde3174fe799600bb0f3cae233d0bf.zip"],
+          strip_prefix = "abseil-cpp-215105818dfde3174fe799600bb0f3cae233d0bf",
+        )
+
     if not native.existing_rule("zlib"):
         http_archive(
             name = "zlib",