smyrna: use a more appropriate type for 'ComboValuesCount'

smyrna: use a more appropriate type for 'ComboValuesCount'

Squashes two compiler warnings:

  gui/gui.c:94:96: warning: conversion to ‘size_t’ {aka ‘long unsigned
    int’} from ‘int’ may change the sign of the result [-Wsign-conversion]
     94 |   attr[attrcount].ComboValuesCount - 1,
        |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
  gui/gui.c:95:78: warning: conversion to ‘size_t’ {aka ‘long unsigned
    int’} from ‘int’ may change the sign of the result [-Wsign-conversion]
     95 |   attr[attrcount].ComboValuesCount,
        |   ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~