Mark plat_vidapi() argument as const and remove the NULL

* plat_vidapi() accepts char pointer, which never gets modifed
later on. Mark it as const.

* In src/config.c, va_name is initialized as NULL, however,
plat_vidapi_name() never returns a NULL pointer, nor it was
initialized under a condition branch. Removing NULL, might
save one instruction, which requires zeroing the register
before setting its initial value.

src/unix/unix_sdl.c

@@ -392,7 +392,7 @@ sdl_reload(void)
 }
 
 int
-plat_vidapi(char *api)
+plat_vidapi(UNUSED(const char *api))
 {
     return 0;
 }