Import RestrictedBuildTest annotation

Adds a line to import the RestrictedBuildTest annotation in
BuildTest.java. This line was removed as part of an upstream revert,
which was causing the CTS (and docs) builds to fail.

Test: m -j cts succeeds
Change-Id: Iab84490d3552def589252730390b2b1d6b16183c
diff --git a/tests/tests/os/src/android/os/cts/BuildTest.java b/tests/tests/os/src/android/os/cts/BuildTest.java
index 09bb7cf..c9650bd 100644
--- a/tests/tests/os/src/android/os/cts/BuildTest.java
+++ b/tests/tests/os/src/android/os/cts/BuildTest.java
@@ -19,6 +19,7 @@
 
 import android.os.Build;
 import android.os.SystemProperties;
+import android.platform.test.annotations.RestrictedBuildTest;
 
 import dalvik.system.VMRuntime;