ART: Fix incorrect logging

Print the string, not the pointer.

Bug: 65574359
Test: m test-art-host
Test: manual
Change-Id: I6031de8b74359844584f8ca90ad91be665148f7a
diff --git a/runtime/java_vm_ext.cc b/runtime/java_vm_ext.cc
index 267f9fd..1593577 100644
--- a/runtime/java_vm_ext.cc
+++ b/runtime/java_vm_ext.cc
@@ -836,7 +836,7 @@
       StringAppendF(error_msg, "Shared library \"%s\" already opened by "
           "ClassLoader %p; can't open in ClassLoader %p",
           path.c_str(), library->GetClassLoader(), class_loader);
-      LOG(WARNING) << error_msg;
+      LOG(WARNING) << *error_msg;
       return false;
     }
     VLOG(jni) << "[Shared library \"" << path << "\" already loaded in "