net: fix missing include of qapi/error.h in netmap.c

The netmap.c file fails to build on FreeBSD with

net/netmap.c:95:9: warning: implicit declaration of function 'error_setg_errno' is invalid in C99 [-Wimplicit-function-declaration]
     error_setg_errno(errp, errno, "Failed to nm_open() %s",
     ^
net/netmap.c:432:9: warning: implicit declaration of function 'error_propagate' is invalid in C99 [-Wimplicit-function-declaration]
     error_propagate(errp, err);
     ^

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1459429690-6144-1-git-send-email-berrange@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
