examples/netmap_compat: fix build on Suse 11

examples/netmap_compat: fix build on Suse 11

 examples/netmap_compat/bridge/../lib/compat_netmap.c:880:41:
 error: ‘POLLRDNORM’ undeclared (first use in this function)

The root casue is POLLRDNORM is more-or-less nonstandard, and it
depends on macro "__USE_XOPEN".

In suse11 sp3, POLLRDNORM will not be defined because "__USE_XOPEN"
issue.

This patch add check if it is not defined, define it.

Fixes: 06371afe394d (examples/netmap_compat: import netmap compatibility example)

Signed-off-by: Michael Qiu <michael.qiu@intel.com>