www/varnish-libvmod-fileserver: fix build on ARM / POWER

Fix build with unsigned char instead signed char:
133 | iface.ifr_name[i] = c as i8;

|         -----------------   ^^^^^^^ expected `u8`, found `i8`

(cherry picked from commit 7eb89cf86e6b50ca33c43a9e3c733bbab131d0b0)