DF-2738 fix compile-verification (guest /usr/obj/usr/src/sys/X86_64_GENERIC,
stock kernel CFLAGS incl. -Werror), after applying fix.diff to the guest
/usr/src copy (reverted afterwards, md5-verified):

$ cd /usr/obj/usr/src/sys/X86_64_GENERIC && make -j4 kern_sysctl.o && make -j4 kern_proc.o
--- kern_sysctl.o ---
cc -c -O2 -pipe -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes
   -Wmissing-prototypes -Wpointer-arith -Wcast-qual -Wold-style-definition
   -Wmissing-include-dirs -Wno-pointer-sign -Winit-self -Wundef
   -Wno-unused-but-set-variable -std=gnu11 -g -nostdinc ... -D_KERNEL
   -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h ... -Werror
   /usr/src/sys/kern/kern_sysctl.c -Werror
--- kern_proc.o ---
cc -c -O2 -pipe ... /usr/src/sys/kern/kern_proc.c -Werror
OBJ-BUILD-OK

(no warnings, no errors; exit 0)

Revert:
$ cd /usr/src && patch -R -p1 < /tmp/fix.diff
done
$ md5 sys/kern/kern_sysctl.c
MD5 (sys/kern/kern_sysctl.c) = 61079cb126100fde1158c8eae27e4fe8
(audit tree sys/kern/kern_sysctl.c md5: 61079cb126100fde1158c8eae27e4fe8 -- pristine match)