sys/nfsserver/nfs_srvkrpc.c doesn't build with KGSSAPI

I just noticed that sys/nfsserver/nfs_srvkrpc.c doesn't build when
options KGSSAPI is specified.

"hostname" is undefined

This appears to be because it is now inside:

#ifdef VIMAGE_GLOBALS
...
#endif
in sys/sys/kernel.h.

I figured I'd let someone more familiar with what VIMAGE_GLOBALS is used
for decide how best to fix this.

rick