make test: build test/ext when doing coverage

This makes VAPI/VOM tests part of code coverage report.

Change-Id: I66511b0a59880c2a64daecd37088268bdf5e1269
Signed-off-by: Klement Sekera <ksekera@cisco.com>
diff --git a/test/Makefile b/test/Makefile
index a2a46d6..3bd8bd0 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -172,7 +172,7 @@
 wipe-doc:
 	@make -C doc wipe BR=$(BR)
 
-cov: wipe-cov reset verify-python-path $(PAPI_INSTALL_DONE)
+cov: wipe-cov reset ext verify-python-path $(PAPI_INSTALL_DONE)
 	@lcov --zerocounters --directory $(VPP_TEST_BUILD_DIR)
 	@test -z "$(EXTERN_COV_DIR)" || lcov --zerocounters --directory $(EXTERN_COV_DIR)
 	$(call retest-func)