Revert "jenkins.sh: disable parallel build"

This reverts commit a352642587d8835deb3c6f55da7986f427835157.

I've confused the libasn1c and asn1c repositories. libasn1c does not
need this workaround, asn1c does.

Related Change-Id: I80eecc60b96e87710218089dfdd1a3f87b40a8b1 (asn1c)
Change-Id: I73eac905524688939e133b6f91ad85514dc0393a
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index dbdfda6..7d68736 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -14,10 +14,7 @@
 
 autoreconf --install --force
 ./configure --enable-werror
-
-# Parallel make is disabled due to a race condition. On Jenkins, it often
-# aborts the build with: "asn1p_y.y:357:13: error: 'param' undeclared"
-$MAKE -j1
+$MAKE $PARALLEL_MAKE
 $MAKE distcheck \
   || cat-testlogs.sh