Fix missing dep (absl-py) for server

Author: wwwillchen

mesop/server/BUILD

@@ -1,4 +1,4 @@
-load("//build_defs:defaults.bzl", "THIRD_PARTY_PY_FLASK", "py_library")
+load("//build_defs:defaults.bzl", "THIRD_PARTY_PY_ABSL_PY", "THIRD_PARTY_PY_FLASK", "py_library")
 
 package(
     default_visibility = ["//build_defs:mesop_internal"],
@@ -11,7 +11,7 @@ py_library(
         "//mesop/editor",
         "//mesop/protos:ui_py_pb2",
         "//mesop/utils",
-    ] + THIRD_PARTY_PY_FLASK,
+    ] + THIRD_PARTY_PY_ABSL_PY + THIRD_PARTY_PY_FLASK,
     # Note: prod_server.py has a runtime dependency on
     # //mesop/web/src/app/prod:web_package, so you must include it
     # in the consuming py_binary's data attribute.