glxext: don't try zink if not enabled in mesa

Commit 7d9ea77b45 ("glx: add automatic zink fallback loading between hw and sw drivers")
added an automatic zink fallback even when the zink gallium is not
enabled at build time.

It leads to unexpected error log while loading drisw driver and
zink is not installed on the rootfs:

  MESA-LOADER: failed to open zink: /usr/lib/dri/zink_dri.so

Fixes: 7d9ea77b45 ("glx: add automatic zink fallback loading between hw and sw drivers")

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Reviewed-by: Antoine Coutant <antoine.coutant@smile.fr>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27478>

src/glx/glxext.c

@@ -919,9 +919,11 @@ __glXInitialize(Display * dpy)
 #endif /* HAVE_DRI3 */
       if (!debug_get_bool_option("LIBGL_DRI2_DISABLE", false))
          dpyPriv->dri2Display = dri2CreateDisplay(dpy);
+#if defined(HAVE_ZINK)
       if (!dpyPriv->dri3Display && !dpyPriv->dri2Display)
          try_zink = !debug_get_bool_option("LIBGL_KOPPER_DISABLE", false) &&
                     !getenv("GALLIUM_DRIVER");
+#endif /* HAVE_ZINK */
    }
 #endif /* GLX_USE_DRM */
    if (glx_direct)