Do not complain if optional mce-plugin-libhybris is not installed

Still emits diagnostic message if mce was started in verbose mode.

mce-hybris.c

@@ -318,6 +318,9 @@ static void *mce_hybris_lookup_function(const char *name)
     if( !(path = mce_hybris_module_path()) ) {
       mce_log(LL_WARN, "could not locate hybris plugin");
     }
+    else if( access(path, F_OK) == -1 && errno == ENOENT ) {
+      mce_log(LL_NOTICE, "%s: not installed", path);
+    }
     else if( !(base = dlopen(path, RTLD_NOW|RTLD_LOCAL|RTLD_DEEPBIND)) ) {
       mce_log(LL_WARN, "%s: failed to load: %s", path, dlerror());
     }