jenkins.sh: use PARALLEL_MAKE to accelerate build speed

Change-Id: Idb75e028d52803e3f8685d42c584f8f96613ec3c
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 35edd37..b4ee280 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -9,5 +9,5 @@
 cd $TOPDIR/sysmoOCTSIM
 cd gcc
 make clean
-make
+make $PARALLEL_MAKE
 make clean