fix boring C lint in header
fix boring C lint in header

With warnings turned sufficiently high in gcc, I get:
  .../vpi_user.h:568: warning: function declaration isn't a prototype
when building VPI modules.  Patch is trivial.

CastMi/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog