[sonic-slave]: Add gmock for sonic-swss-common tests (#8950) (#12461)

Why I did it
Cherry-pick from #8950

Sonic-swss-common requires gmock for staged unit tests

How I did it
Installed dependency in sonic-buster-build docker file

How to verify it
rebuild the build container

Author: xumia

sonic-slave-buster/Dockerfile.j2

@@ -254,6 +254,8 @@ RUN apt-get update && apt-get install -y \
 # For gtest
         libgtest-dev \
         cmake \
+# For gmock
+        libgmock-dev \
 # For pam_tacplus build
         autoconf-archive \
 # For iproute2