build: provide virNetDevSysfsFile on non-Linux

Commit 49ed6cff is broken on mingw and other non-linux platforms:

  CCLD     libvirt.la
  Cannot export virNetDevSysfsFile: symbol not defined
  collect2: error: ld returned 1 exit status

* src/util/virnetdev.c: Provide virNetDevSysfsFile fallback.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virnetdev.c

@@ -1877,6 +1877,17 @@ virNetDevGetVirtualFunctionInfo(const char *vfname ATTRIBUTE_UNUSED,
                          _("Unable to get virtual function info on this platform"));
     return -1;
 }
+
+int
+virNetDevSysfsFile(char **pf_sysfs_device_link ATTRIBUTE_UNUSED,
+                   const char *ifname ATTRIBUTE_UNUSED,
+                   const char *file ATTRIBUTE_UNUSED)
+{
+    virReportSystemError(ENOSYS, "%s",
+                         _("Unable to get sysfs info on this platform"));
+    return -1;
+}
+
 #endif /* !__linux__ */
 #if defined(__linux__) && defined(HAVE_LIBNL) && defined(IFLA_VF_MAX)