Allow opentitan-based DV test code visable to out-of-tree build

opentitan_functest can't support airgapped build. Export the source code
files so the artifacts can be built out of the repo

Change-Id: Ie248136397804c391a5972dbcf93a1b81f7821c8

sw/device/tests/sim_dv/BUILD

diff --git a/sw/device/tests/sim_dv/BUILD b/sw/device/tests/sim_dv/BUILD
index 9bbed52..f52b107 100644
--- a/sw/device/tests/sim_dv/BUILD
+++ b/sw/device/tests/sim_dv/BUILD
@@ -5,6 +5,10 @@
 load("//rules:opentitan.bzl", "OPENTITAN_CPU")
 load("//rules:opentitan_test.bzl", "opentitan_functest")
 
+# Export source code files so we can build DV test artifacts in matcha airgapped
+# environment.
+exports_files(glob(["*.c"]))
+
 opentitan_functest(
     name = "uart_tx_rx_test",
     srcs = ["uart_tx_rx_test.c"],