Add external/conscrypt to visibility

`conscrypt` has a `required` property set to boringssl_self_test.
Add the package where `conscrypt` is located to the visibility property
of the test module.

This is in preparation for an upcoming change in Soong which handles the
installation of required modules.

Bug: 321626681
Test: builds

Change-Id: Ie97f937d7408f0221e73ae3843a23bce5610817a
diff --git a/selftest/Android.bp b/selftest/Android.bp
index d746daf..c215676 100644
--- a/selftest/Android.bp
+++ b/selftest/Android.bp
@@ -45,6 +45,7 @@
     name: "boringssl_self_test",
     visibility: [
         "//art/build/sdk",
+        "//external/conscrypt",
         "//external/conscrypt/apex",
         "//external/conscrypt/apex/testing",
     ],