[Chromecast] Add header for int64 in capture_service constants.h

A defined struct uses int64_t but didn't include cstdint, which causes
issue if some class doens't have cstdint included but have constants.h
included.

Bug: NONE.
Test: NONE.
Change-Id: I0a1700fac5c4a91735f889514d95c9a03b59edda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986013
Commit-Queue: Kehuang Li <kehuangli@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Auto-Submit: Kehuang Li <kehuangli@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728026}

chromecast/media/audio/capture_service/constants.h

@@ -5,6 +5,8 @@
 #ifndef CHROMECAST_MEDIA_AUDIO_CAPTURE_SERVICE_CONSTANTS_H_
 #define CHROMECAST_MEDIA_AUDIO_CAPTURE_SERVICE_CONSTANTS_H_
 
+#include <cstdint>
+
 namespace chromecast {
 namespace media {
 namespace capture_service {