Missing #include <sys/eventhandler.h> in sys/net/route/nhop_ctl.c

Change d05d1f2560823 introduces this build error with my custom kernel 
config:

/usr/src/sys/net/route/nhop_ctl.c:114:2: error: call to
       undeclared function 'EVENTHANDLER_REGISTER'; ISO C99 and later do not
       support implicit function declarations
       [-Werror,-Wimplicit-function-declaration]
   114 |         EVENTHANDLER_REGISTER(ifnet_event, nhops_ifnet_event, NULL,
       |         ^
/usr/src/sys/net/route/nhop_ctl.c:114:24: error: use of
       undeclared identifier 'ifnet_event'
   114 |         EVENTHANDLER_REGISTER(ifnet_event, nhops_ifnet_event, NULL,
       |                               ^~~~~~~~~~~
/usr/src/sys/net/route/nhop_ctl.c:115:6: error: use of
       undeclared identifier 'EVENTHANDLER_PRI_ANY'
   115 |             EVENTHANDLER_PRI_ANY);
       |             ^~~~~~~~~~~~~~~~~~~~
/usr/src/sys/net/route/nhop_ctl.c:116:24: error: use of
       undeclared identifier 'ifnet_link_event'
   116 |         EVENTHANDLER_REGISTER(ifnet_link_event, 
nhops_ifnet_link_event, NULL,
       |                               ^~~~~~~~~~~~~~~~
/usr/src/sys/net/route/nhop_ctl.c:117:6: error: use of
       undeclared identifier 'EVENTHANDLER_PRI_ANY'
   117 |             EVENTHANDLER_PRI_ANY);
       |             ^~~~~~~~~~~~~~~~~~~~

Adding the missing include resolves the error.

Ian