Fix BSD and win32 builds

  CC    net/tap-bsd.o
/src/qemu/net/tap-bsd.c: In function `tap_open':
/src/qemu/net/tap-bsd.c:93: warning: implicit declaration of function `error_report'

  CC    sparc-softmmu/../net/tap-win32.o
cc1: warnings being treated as errors
/src/qemu/target-sparc/../net/tap-win32.c: In function 'net_init_tap':
/src/qemu/target-sparc/../net/tap-win32.c:709: warning: implicit declaration of function 'error_report'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>