Fix compiler warnings

Starting with commit df7a86ed735eafefbd046c8cad7134652fe3f600,
mingw32 builds result in a compiler warning for dns_addr:

  CC    slirp/slirp.o
/home/stefan/src/qemu/savannah/qemu/slirp/slirp.c:50: warning: missing braces around initializer
/home/stefan/src/qemu/savannah/qemu/slirp/slirp.c:50: warning: (near initialization for ‘dns_addr.S_un’)

Removing the assignment fixes the warning without the need of special code
for mingw32 (and also saves some bytes in the resulting binary).

To fix another potential compiler warning, the missing 'static'
attribute was added.

The same changes were applied to dns_addr_time.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
