build: skip virportallocatortest on cygwin

Cygwin supports <dlfcn.h> and even has limited LD_PRELOAD
capabilities; but because it does not use ELF binaries it
cannot support RTLD_NEXT lookups.

  CC       libvirportallocatormock_la-virportallocatortest.lo
virportallocatortest.c: In function 'init_syms':
virportallocatortest.c:47:24: error: 'RTLD_NEXT' undeclared (first use in this function)
     realsocket = dlsym(RTLD_NEXT, "socket");

* tests/virportallocatortest.c: Also require RTLD_NEXT.
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/virportallocatortest.c

@@ -23,7 +23,7 @@
 #include "virfile.h"
 #include "testutils.h"
 
-#if HAVE_DLFCN_H
+#if HAVE_DLFCN_H && defined(RTLD_NEXT)
 
 # ifdef MOCK_HELPER
 #  include "internal.h"