vlan: Fix musl build error

vlan: Fix musl build error

caddr_t is legacy BSD and should be avoided [1]. While glibc may still
use __caddr_t as the type, Linux kernel does not (it is "void __user *
ifru_data").

This fixes compile errors with the musl libc:

../src/ap/vlan_init.c: In function 'br_delif':
../src/ap/vlan_init.c:218:18: error: '__caddr_t' undeclared (first use in this function)
  ifr.ifr_data = (__caddr_t) args;

[1] http://stackoverflow.com/questions/6381526/what-is-the-significance-of-caddr-t-and-when-is-it-used

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>