build: add libssl-dev for ubuntu 16.04 and 18.04

The recent changes to Makefile lead to the lack of libssl-dev
dependency for ubuntu 16.04 and 18.04. Add libssl-dev to DEB_DEPENDS
variable for corresponding ubuntu version.

Type: fix

Change-Id: I42e0e4761d5ec377de71b11cccf747c7f55ca337
Signed-off-by: Jieqiang Wang <jieqiang.wang@arm.com>
diff --git a/Makefile b/Makefile
index f69a688..545ccdb 100644
--- a/Makefile
+++ b/Makefile
@@ -75,8 +75,10 @@
  
 ifeq ($(OS_VERSION_ID),16.04)
 	DEB_DEPENDS += python-dev
+	DEB_DEPENDS += libssl-dev
 else ifeq ($(OS_VERSION_ID),18.04)
 	DEB_DEPENDS += python-dev
+	DEB_DEPENDS += libssl-dev
 else ifeq ($(OS_ID)-$(OS_VERSION_ID),debian-8)
 	DEB_DEPENDS += libssl-dev
 	APT_ARGS = -t jessie-backports