[libc++] Fix modules and benchmarks CI builds when incomplete features are disabled

Differential Revision: https://reviews.llvm.org/D119036

NOKEYCHECK=True
GitOrigin-RevId: 506cf6dc048835c598b654e43ed8f723a42e39ba
diff --git a/include/module.modulemap b/include/module.modulemap
index a182ded..442958b 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -2,7 +2,8 @@
 // since __config may be included from C headers which may create an
 // include cycle.
 module std_config [system] [extern_c] {
-    header "__config"
+  textual header "__config"
+  textual header "__config_site"
 }
 
 module std [system] {