build: fix include path for cygwin

build: fix include path for cygwin

Without this, cygwin failed to compile:

In file included from ../src/rpc/virnetmessage.h:24,
                 from ../src/rpc/virnetclient.h:27,
                 from remote/remote_driver.c:31:
../src/rpc/virnetprotocol.h:9:21: error: rpc/rpc.h: No such file or directory

With that fixed, compilation warned:

rpc/virnetsocket.c: In function 'virNetSocketNewListenUNIX':
rpc/virnetsocket.c:347: warning: format '%d' expects type 'int', but argument 8 has type 'gid_t' [-Wformat]
rpc/virnetsocket.c: In function 'virNetSocketGetLocalIdentity':
rpc/virnetsocket.c:743: warning: pointer targets in passing argument 5 of 'getsockopt' differ in signedness

* src/Makefile.am (libvirt_driver_remote_la_CFLAGS)
(libvirt_net_rpc_client_la_CFLAGS)
(libvirt_net_rpc_server_la_CFLAGS): Include XDR_CFLAGS, for rpc
headers on cygwin.
* src/rpc/virnetsocket.c (virNetSocketNewListenUNIX)
(virNetSocketGetLocalIdentity): Avoid compiler warnings.