fou: make local function static

The build header functions are not used by any other code.

net/ipv6/fou6.c:36:5: warning: no previous prototype for ‘fou6_build_header’ [-Wmissing-prototypes]
net/ipv6/fou6.c:54:5: warning: no previous prototype for ‘gue6_build_header’ [-Wmissing-prototypes]

Need to do some code rearranging to satisfy different Kconfig possiblities.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>