build: curvetun: link against sysctl.o

build: curvetun: link against sysctl.o

Fixes curvetun build failure due to undefined reference to
sysctl_get_int in sock.o.

  [...]
    LD	curvetun
  curvetun/sock.o: In function `set_system_socket_mem.part.0':
  sock.c:(.text+0xc0): undefined reference to `sysctl_set_int'
  curvetun/sock.o: In function `set_system_socket_memory':
  sock.c:(.text+0x4dd): undefined reference to `sysctl_get_int'
  sock.c:(.text+0x505): undefined reference to `sysctl_get_int'
  sock.c:(.text+0x52e): undefined reference to `sysctl_get_int'
  sock.c:(.text+0x54f): undefined reference to `sysctl_get_int'
  collect2: error: ld returned 1 exit status

Signed-off-by: Tobias Geerinckx-Rice <tobias.geerinckx.rice@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>