cmSymTbl.c: Fixed cmSymTblRegisterFmt() to return new symbol id.

cmSymTbl.c

 {
   va_list vl;
   va_start(vl,fmt);
-  cmSymTblRegisterVFmt(h,fmt,vl);
+  unsigned id = cmSymTblRegisterVFmt(h,fmt,vl);
   va_end(vl);
+  return id;
 }