efi-loader: Add @ to valid characters

This is now a valid character with the introduction of multi UKI
profiles, so update the function to allow it.

src/shared/efi-loader.c

@@ -364,5 +364,5 @@ bool efi_loader_entry_name_valid(const char *s) {
         if (!filename_is_valid(s)) /* Make sure entry names fit in filenames */
                 return false;
 
-        return in_charset(s, ALPHANUMERICAL "+-_.");
+        return in_charset(s, ALPHANUMERICAL "+-_.@");
 }