fix string literal in form_wrap.c so that ncurses.so will build

the gem wouldn't build because of the following errors

```
{13-06-12 23:01}raxcity:~/ncurses-ruby/ext/ncurses@master✗✗✗✗✗✗ jeff# make
compiling menu_wrap.c
compiling form_wrap.c
form_wrap.c: In function 'make_arg':
form_wrap.c:1133:5: error: format not a string literal and no format arguments [-Werror=format-security]
cc1: some warnings being treated as errors
make: *** [form_wrap.o] Error 1
```
This patch makes the format a string literal with arguments, allowing it to build.