Export gki/testdata/testkey_rsa4096.pem to Bazel rules.

Used to build boot images in Bazel rules.

Test: TH
Bug: 233730966
Change-Id: Id9b506db6b57bcf3610e883f64f461b5635697d1
diff --git a/BUILD.bazel b/BUILD.bazel
index e80a82a..6fc323b 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -14,4 +14,5 @@
 
 exports_files([
     "mkbootimg.py",
+    "gki/testdata/testkey_rsa4096.pem",
 ])