build: Skip check-export-symbol on Windows

build: Skip check-export-symbol on Windows

This patch makes automake skip the function 'check-export-symbol' under
Windows.

The main reason is that msys does not have an implementation of rev
used by:
https://github.com/openvswitch/ovs/blob/master/datapath/Makefile.am#L34

and can be seen on:
https://ci.appveyor.com/project/blp/ovs/build/1.0.1851#L322

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Co-authored-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>