build: fix build on Cygwin

build: fix build on Cygwin

The RPC fixups needed on Linux are also needed on cygwin, and
worked without further tweaking to the list of fixups.  Also,
unlike BSD, Cygwin exports 'struct ifreq', but unlike Linux,
Cygwin lacks the ioctls that we were using 'struct ifreq' to
access.  This patch allows compilation under cygwin.

* src/rpc/genprotocol.pl: Also perform fixups on cygwin.
* src/util/virnetdev.c (HAVE_STRUCT_IFREQ): Also require AF_PACKET
definition.
* src/util/virnetdevbridge.c (virNetDevSetupControlFull): Only
compile if SIOCBRADDBR works.