kconfig: make input_mode static

Sparse reports:
  warning: symbol 'input_mode' was not declared. Should it be static?
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/conf.c

@@ -35,7 +35,8 @@ enum input_mode {
 	savedefconfig,
 	listnewconfig,
 	olddefconfig,
-} input_mode = oldaskconfig;
+};
+static enum input_mode input_mode = oldaskconfig;
 
 static int indent = 1;
 static int tty_stdio;