math/fityk: unbreak build on 8.x

r385070 exposed another issue:

In file included from ceria.cpp:4:0:
ceria.h:52:23: error: 'FILE' was not declared in this scope
CelFile read_cel_file(FILE* f);

^

ceria.h:52:29: error: 'f' was not declared in this scope
CelFile read_cel_file(FILE* f);

^

ceria.h:53:41: error: 'FILE' has not been declared
void write_cel_file(CelFile const& cel, FILE* f);

^

ceria.cpp: In function 'CelFile read_cel_file(FILE*)':
ceria.cpp:719:30: error: 'CelFile read_cel_file(FILE*)' redeclared as different kind of symbol
CelFile read_cel_file(FILE *f)

^

In file included from ceria.cpp:4:0:
ceria.h:52:9: error: previous declaration of 'CelFile read_cel_file'
CelFile read_cel_file(FILE* f);

^

Reported by: pkg-fallout
Approved by: portmgr blanket