[gdb/build] Fix build without ncurses in maintenance_info_screen

With a build without ncurses we run into:
...
src/gdb/utils.c: In function ‘void maintenance_info_screen(const char*, int)’:
src/gdb/utils.c:1310:7: error: ‘COLS’ was not declared in this scope
       COLS);
       ^~~~
src/gdb/utils.c:1331:8: error: ‘LINES’ was not declared in this scope
        LINES);
        ^~~~~
...

Fix this by using HAVE_LIBCURSES.

Tested on x86_64-linux.

PR build/30391
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30391

gdb/utils.c

@@ -1304,10 +1304,12 @@ maintenance_info_screen (const char *args, int from_tty)
 		  ? " (unlimited - 1)"
 		  : "")));
 
+#ifdef HAVE_LIBCURSES
   gdb_printf (gdb_stdout,
 	     _("Number of characters curses thinks "
 	       "are in a line is %d.\n"),
 	     COLS);
+#endif
 
   gdb_printf (gdb_stdout,
 	      _("Number of characters environment thinks "
@@ -1325,10 +1327,12 @@ maintenance_info_screen (const char *args, int from_tty)
 	      rows,
 	      rows == sqrt_int_max ? " (unlimited)" : "");
 
+#ifdef HAVE_LIBCURSES
   gdb_printf (gdb_stdout,
 	     _("Number of lines curses thinks "
 	       "are in a page is %d.\n"),
 	      LINES);
+#endif
 
   gdb_printf (gdb_stdout,
 	      _("Number of lines environment thinks "