buck2: shim: unused lint (#3773)

Summary:
Pull Request resolved: #3773

broken in D57915321

Error on `buck2-oss` job P1383482720

Reviewed By: JakobDegen

Differential Revision: D57953492

fbshipit-source-id: 4efb37a538136f935e03c8734b871413d392833c

Author: iguridi

shim/shims.bzl

@@ -137,7 +137,7 @@ def cpp_library(
         private_headers = None,
         propagated_pp_flags = (),
         **kwargs):
-    _unused = (undefined_symbols, arch_preprocessor_flags, modular_headers, arch_compiler_flags, tags)  # @unused
+    _unused = (undefined_symbols, arch_preprocessor_flags, modular_headers, arch_compiler_flags, tags, propagated_pp_flags)  # @unused
     if os_deps:
         deps += _select_os_deps(_fix_dict_deps(os_deps))
     if headers == None: