build: silence stderr on coverity rules

We try to execute git in order to get the dependencies for the coverity
rules. And it gets executed even when we are not calling that specific
rule.  Later we may want to improve it, but for now let's just silence
the errors of not being a git repository when executing this on a
packaged version.
diff --git a/Makefile.am b/Makefile.am
index 5614317..a1b6632 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -451,9 +451,9 @@
 	cov-build --dir cov-int make -j 4
 	tar caf $@ cov-int
 
-coverity-tar: kmod-coverity-$(shell git describe).tar.xz
+coverity-tar: kmod-coverity-$(shell git describe  2>/dev/null).tar.xz
 
-coverity-sync: kmod-coverity-$(shell git describe).tar.xz
+coverity-sync: kmod-coverity-$(shell git describe 2>/dev/null).tar.xz
 	@echo "uploading coverity tarball"
 	@curl --form token=$(COVERITY_KMOD_TOKEN) \
 		--form email=lucas.de.marchi@gmail.com \