compile with gcc-14 fails due to `-Werror=incompatible-pointer-types`

Patch provided by Mamoru Tasaka.

src/tclspice.c

@@ -2433,7 +2433,7 @@ tmeasure TCL_CMDPROCARGS(clientData, interp, argc, argv)
         return TCL_ERROR;
     }
 
-    wl = wl_build((char **)argv);
+    wl = wl_build((const char * const *)argv);
 
     get_measure2(wl, &mvalue, NULL, FALSE);