[Minor] fix compilation without replxx

[Minor] fix compilation without replxx

../src/rspamadm/lua_repl.c: In function ‘rspamadm_lua_run_repl’:
../src/rspamadm/lua_repl.c:615:8: warning: unused variable ‘i’ [-Wunused-variable]
  615 |  gsize i;
      |        ^
../src/rspamadm/lua_repl.c:614:11: warning: unused variable ‘tb’ [-Wunused-variable]
  614 |  GString *tb = NULL;
      |           ^~
../src/rspamadm/lua_repl.c:613:11: warning: unused variable ‘is_multiline’ [-Wunused-variable]
  613 |  gboolean is_multiline = FALSE;
      |           ^~~~~~~~~~~~
../src/rspamadm/lua_repl.c: In function ‘rspamadm_lua’:
../src/rspamadm/lua_repl.c:1007:3: warning: implicit declaration of function ‘replxx_set_max_history_size’ [-Wimplicit-function-declaration]
 1007 |   replxx_set_max_history_size (rx_instance, max_history);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/rspamadm/lua_repl.c:1007:32: error: ‘rx_instance’ undeclared (first use in this function)
 1007 |   replxx_set_max_history_size (rx_instance, max_history);
      |                                ^~~~~~~~~~~
../src/rspamadm/lua_repl.c:1007:32: note: each undeclared identifier is reported only once for each function it appears in
../src/rspamadm/lua_repl.c:1008:3: warning: implicit declaration of function ‘replxx_history_load’ [-Wimplicit-function-declaration]
 1008 |   replxx_history_load (rx_instance, histfile);
      |   ^~~~~~~~~~~~~~~~~~~
../src/rspamadm/lua_repl.c:1010:3: warning: implicit declaration of function ‘replxx_history_save’ [-Wimplicit-function-declaration]
 1010 |   replxx_history_save (rx_instance, histfile);
      |   ^~~~~~~~~~~~~~~~~~~