[Build]: Fix gmock not found issue in debian stretch slave container (sonic-net#9669)

Fix the gmock not found issue in debian stretch slave container.
Some of platforms, such as naphos, innovium, have such issue.

Author: xumia

sonic-slave-stretch/Dockerfile.j2

@@ -300,6 +300,9 @@ RUN apt-get update && apt-get install -y \
         libauparse-dev \
         auditd
 
+# For gmock
+RUN apt-get install -y libgmock-dev -t stretch-backports
+
 # Install dependencies for dhcp relay test 
 RUN pip3 install parameterized==0.8.1
 RUN pip3 install pyfakefs