fix -Wstrict-prototypes warning

fix -Wstrict-prototypes warning

function declarations should always explicitly say no argument is expected,
as in C 'foo bar()' means bar can take any parameter.

This was noticed when building another project with -Wall -Wextra and getting
the following warning on installed header:
/usr/include/zck.h:66:5: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
   66 |     __attribute__ ((warn_unused_result));
      |     ^~~~~~~~~~~~~
/usr/include/zck.h:325:5: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  325 |     __attribute__ ((warn_unused_result));
      |     ^~~~~~~~~~~~~