Make astc_encoder optional

The `astc_encoder` dependency should be optional as
its not enabled when built with Chromium:
https://chromium.googlesource.com/angle/angle/+/refs/heads/main/gni/angle.gni#193

Bug: angleproject:7687
Change-Id: Ie90b5f9b41b27d8ea064c28d20299bc84a803339
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3914175
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
diff --git a/DEPS b/DEPS
index bffb2c8..49b8b7d 100644
--- a/DEPS
+++ b/DEPS
@@ -331,6 +331,7 @@
 
   'third_party/astc-encoder/src': {
     'url': '{chromium_git}/external/github.com/ARM-software/astc-encoder@573c475389bf51d16a5c3fc8348092e094e50e8f',
+    'condition': 'not build_with_chromium',
   },
 
   'third_party/bazel': {