Use thiserror in pVM firmware.

Test: m pvmfw
Change-Id: Ib98435fadff21754166b9187941a3e2d265082e6

diff --git a/libs/libhypervisor_backends/rules.mk b/libs/libhypervisor_backends/rules.mk
index 6fc9dea..1a48773 100644
--- a/libs/libhypervisor_backends/rules.mk
+++ b/libs/libhypervisor_backends/rules.mk
@@ -8,6 +8,7 @@
 	trusty/user/base/lib/liballoc-rust \
 	$(call FIND_CRATE,once_cell) \
 	$(call FIND_CRATE,smccc) \
+	$(call FIND_CRATE,thiserror) \
 	$(call FIND_CRATE,uuid) \
 
 include make/library.mk
\ No newline at end of file