Since update to glibc-2.26 removed the /usr/include/rpc/rpc.h we used until now,
it showed us a problem with not using XDR_CFLAGS properly.  On linux that
variable has usually -I/usr/include/tirpc because we already probe for it
properly, we just don't use it everywhere we need.  It is needed by wireshark
dissector as well as testutilsqemu.c (through includes) so the build fails with:

wireshark/src/packet-libvirt.c:33:10: fatal error: rpc/xdr.h: No such file or directory
 #include <rpc/xdr.h>
          ^~~~~~~~~~~

and

In file included from ../src/logging/log_manager.h:29:0,
                 from ../src/qemu/qemu_domain.h:40,
                 from testutilsqemu.c:11:
../src/logging/log_protocol.h:9:10: fatal error: rpc/rpc.h: No such file or directory
 #include <rpc/rpc.h>
          ^~~~~~~~~~~

Since lot of tests use testutilsqemu.c it is easier to add XDR_CFLAGS to
AM_CFLAGS than adding it to all $binary_CFLAGS.  It's just for tests and we
already have bunch of CFLAGS there anyway.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>