Make apex availability of some bionic libraries explicit

The libraries are
- libdl_static
- liblinker_main
- liblinker_malloc
- libsystemproperties

The availability to runtime apex was done implicitly using a baseline map in
build/soong/apex/apex.go. Make this explicit in Android.bp

Bug: 281077552
Test: m nothing
Change-Id: I029ae204f6cfef8c301a20b7c4294636b60b38be
diff --git a/libc/system_properties/Android.bp b/libc/system_properties/Android.bp
index af8bda9..a9bdeb5 100644
--- a/libc/system_properties/Android.bp
+++ b/libc/system_properties/Android.bp
@@ -32,6 +32,9 @@
         "bionic/libstdc++/include",
     ],
     export_include_dirs: ["include"],
+    apex_available: [
+        "com.android.runtime",
+    ],
 }
 
 cc_benchmark {