samples: subsys: ipc: Fix warning of `size_t` formatting with `%.*s`

This addresses the following warning building with `CONFIG_64BIT=y`:

    error: field precision specifier '.*' expects argument of type 'int',
    but argument X has type 'size_t' {aka 'long unsigned int'}

Signed-off-by: Chen Xingyu <hi@xingrz.me>