Revert "Remove unused impl-log4j library"

This reverts commit 96b8bf81abf085c8b1e46db0ce8eb71797ac628d.

Reason for revert: This cause the P0 Issue 15898 on v3.6.0

Release-Notes: skip
Change-Id: I86af69ddbcbab0802f7da0f4f172a280828b8987
diff --git a/lib/log/BUILD b/lib/log/BUILD
index 6a85bd1..21c4d47 100644
--- a/lib/log/BUILD
+++ b/lib/log/BUILD
@@ -18,6 +18,14 @@
 )
 
 java_library(
+    name = "impl-log4j",
+    data = ["//lib:LICENSE-slf4j"],
+    visibility = ["//visibility:public"],
+    exports = ["@impl-log4j//jar"],
+    runtime_deps = [":log4j"],
+)
+
+java_library(
     name = "jcl-over-slf4j",
     data = ["//lib:LICENSE-slf4j"],
     visibility = ["//visibility:public"],
diff --git a/tools/bzl/pkg_war.bzl b/tools/bzl/pkg_war.bzl
index 43e9b3e..c9ac0fe 100644
--- a/tools/bzl/pkg_war.bzl
+++ b/tools/bzl/pkg_war.bzl
@@ -22,6 +22,7 @@
     "//lib/bouncycastle:bcpkix",
     "//lib/bouncycastle:bcprov",
     "//lib/bouncycastle:bcpg",
+    "//lib/log:impl-log4j",
     "//prolog:gerrit-prolog-common",
     "//resources:log4j-config",
 ]