Use v4l2 when compiling cast shell

Bug: 982122
Change-Id: I9dbf678992caec7ac97ef028e9ec16c2da7db02c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735523Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684422}

tools/mb/mb_config.pyl

@@ -1972,7 +1972,7 @@
     },
 
     'cast': {
-      'gn_args': 'is_chromecast=true',
+      'gn_args': 'is_chromecast=true use_v4l2=true',
     },
 
     'cast_audio': {