sys/dev/amr build error with Clang

[deeptech71 at gmail.com]

Hey, Baldie:

/usr/src/sys/modules/amr/../../dev/amr/amr.c:970:1: error: function
       'amr_periodic' is not needed and will not be emitted
       [-Werror,-Wunneeded-internal-declaration]
amr_periodic(void *data)
^
1 error generated.
*** [amr.o] Error code 1

Stop in /usr/src/sys/modules/amr.