Makefile: explicitly pass $(BUILD_DIR) to gcovr

Best to be explicit about where to find things.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
diff --git a/Makefile b/Makefile
index 7202ac7..248fe9d 100644
--- a/Makefile
+++ b/Makefile
@@ -964,7 +964,8 @@
 %/coverage-report.html:
 	@mkdir -p $*
 	$(call quiet-command,\
-		gcovr -r $(SRC_PATH) -p --html --html-details -o $@, \
+		gcovr -r $(SRC_PATH) --object-directory $(BUILD_PATH) \
+		-p --html --html-details -o $@, \
 		"GEN", "coverage-report.html")
 
 .PHONY: coverage-report