frag6.c: make compile with gcc

Removing the prototype from the header and making the function static
in r350533 makes architectures using gcc complain "function declaration
isn't a prototype". Add the missing void given the function has no
arguments.

Reported by: the CI machinery
Pointyhat to: bz
MFC after: 3 months
X-MFC with: r350533
Sponsored by: Netflix