net: openvswitch: remove unnecessary linux/genetlink.h include

The only legit reason I could think of for net/genetlink.h
and linux/genetlink.h to be separate would be if one was
included by other headers and we wanted to keep it lightweight.
That is not the case, net/openvswitch/meter.h includes
linux/genetlink.h but for no apparent reason (for struct genl_family
perhaps? it's not necessary, types of externs do not need
to be known).

Link: https://lore.kernel.org/r/20240329175710.291749-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>