Fix this compile-time warning:
rptun/rptun.c:956:22: warning: '%lx' directive output may be truncated writing between 1 and 16 bytes into a region of size 14 [-Wformat-truncation=]
956 | snprintf(arg1, 16, "0x%" PRIxPTR, (uintptr_t)priv);
| ^~~~~
rptun/rptun.c:956:25: note: format string is defined here
956 | snprintf(arg1, 16, "0x%" PRIxPTR, (uintptr_t)priv);
rptun/rptun.c:956:22: note: directive argument in the range [1, 18446744073709551615]
956 | snprintf(arg1, 16, "0x%" PRIxPTR, (uintptr_t)priv);
| ^~~~~
rptun/rptun.c:956:3: note: 'snprintf' output between 4 and 19 bytes into a destination of size 16
956 | snprintf(arg1, 16, "0x%" PRIxPTR, (uintptr_t)priv);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>