Make .whitelisted property "append"-able

.whitelisted is a list of sonames seperated by ":" just like
.shared_libs and .paths properties. It makes sense to make it appendable
using += as well ass .shared_libs and .paths.

Bug: n/a
Test: atest linker-unit-tests
Change-Id: If61cc553c8080e8a58de8a3a6051c1853f7bfe5f
diff --git a/linker/linker_config.cpp b/linker/linker_config.cpp
index ada25a5..aaa3a03 100644
--- a/linker/linker_config.cpp
+++ b/linker/linker_config.cpp
@@ -325,7 +325,8 @@
           value = "," + value;
           (*properties)[name].append_value(std::move(value));
         } else if (android::base::EndsWith(name, ".paths") ||
-                   android::base::EndsWith(name, ".shared_libs")) {
+                   android::base::EndsWith(name, ".shared_libs") ||
+                   android::base::EndsWith(name, ".whitelisted")) {
           value = ":" + value;
           (*properties)[name].append_value(std::move(value));
         } else {