rust: Register rustdoc with checkbuild

Bug: 197237709
Test: m checkbuild, manually verify rustdoc targets built
Change-Id: Icfcc4a8c1bc7de4c5e1abbe495646504490ac927
diff --git a/rust/rust.go b/rust/rust.go
index 300c0f5..66eb38c 100644
--- a/rust/rust.go
+++ b/rust/rust.go
@@ -901,6 +901,9 @@
 		bloaty.MeasureSizeForPaths(ctx, mod.compiler.strippedOutputFilePath(), android.OptionalPathForPath(mod.compiler.unstrippedOutputFilePath()))
 
 		mod.docTimestampFile = mod.compiler.rustdoc(ctx, flags, deps)
+		if mod.docTimestampFile.Valid() {
+			ctx.CheckbuildFile(mod.docTimestampFile.Path())
+		}
 
 		// glob exported headers for snapshot, if BOARD_VNDK_VERSION is current or
 		// RECOVERY_SNAPSHOT_VERSION is current.