Add missing gpg2 flag

According to the man page, the other options are not used unless --batch is passed as well.

Author: TheAssassin

src/appimagetool.c

@@ -1201,6 +1201,7 @@ main (int argc, char *argv[])
                         g_ptr_array_add(sign_process_argv, "0");
                         g_ptr_array_add(sign_process_argv, "--pinentry-mode");
                         g_ptr_array_add(sign_process_argv, "loopback");
+                        g_ptr_array_add(sign_process_argv, "--batch");
                     }
 
                     g_ptr_array_add(sign_process_argv, digestfile);