devlink: add missing install of header

The new devlink.h in uapi was not being installed by
make headers_install
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/Kbuild

@@ -96,6 +96,7 @@ header-y += cyclades.h
 header-y += cycx_cfm.h
 header-y += dcbnl.h
 header-y += dccp.h
+header-y += devlink.h
 header-y += dlmconstants.h
 header-y += dlm_device.h
 header-y += dlm.h