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.
diff --git a/protobuf_deps.bzl b/protobuf_deps.bzl
index 601e6c0..de5d536 100644
--- a/protobuf_deps.bzl
+++ b/protobuf_deps.bzl
@@ -27,6 +27,15 @@
             ],
         )
 
+    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",