genetlink: remove linux/genetlink.h

genetlink.h is a shell of what used to be a combined uAPI
and kernel header over a decade ago. It has fewer than
10 lines of code. Merge it into net/genetlink.h.
In some ways it'd be better to keep the combined header
under linux/ but it would make looking through git history
harder.

Acked-by: Sven Eckelmann <sven@narfation.org>
Link: https://lore.kernel.org/r/20240329175710.291749-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/openvswitch/datapath.c

@@ -15,7 +15,6 @@
 #include <linux/delay.h>
 #include <linux/time.h>
 #include <linux/etherdevice.h>
-#include <linux/genetlink.h>
 #include <linux/kernel.h>
 #include <linux/kthread.h>
 #include <linux/mutex.h>