printk/index: Fix warning about missing prototypes

The commit 33701557 ("printk: Userspace format indexing support")
triggered the following build failure:

kernel/printk/index.c:140:6: warning: no previous prototype for ‘pi_create_file’ [-Wmissing-prototypes]
 void pi_create_file(struct module *mod)
      ^~~~~~~~~~~~~~
kernel/printk/index.c:146:6: warning: no previous prototype for ‘pi_remove_file’ [-Wmissing-prototypes]
 void pi_remove_file(struct module *mod)
      ^~~~~~~~~~~~~~

Fixes: 33701557 ("printk: Userspace format indexing support")
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Chris Down <chris@chrisdown.name>
[pmladek@suse.com: Let the compiler decide about inlining.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/lkml/YPql089IwSpudw%2F1@alley/