coverity: ensure the systemd install directory is /tmp

Because the systemd install directory does not use --prefix as
base. For this reason, the build failed before, because installing
to /lib/systemd failed.

Change-Id: I39b491fc09f0db0efd8d9d09e8f4cba4ba49cdde
diff --git a/coverity/build_Osmocom.sh b/coverity/build_Osmocom.sh
index 920c0b9..86bb569 100755
--- a/coverity/build_Osmocom.sh
+++ b/coverity/build_Osmocom.sh
@@ -12,7 +12,7 @@
 
 do_build() {
 	autoreconf --install --force
-	./configure --prefix="$prefix" $*
+	./configure --prefix="$prefix" --with-systemdsystemunitdir=no $*
 
 	make $PARALLEL_MAKE
 	make install