[docker-syncd-mlnx-rpc]: fix mellanox syncd docker build (#2551)

Signed-off-by: Guohan Lu <gulv@microsoft.com>

platform/mellanox/docker-syncd-mlnx-rpc/Dockerfile.j2

@@ -25,6 +25,7 @@ dpkg_apt debs/{{ deb }}{{'; '}}
 RUN dpkg_apt() { [ -f $1 ] && { dpkg -i $1 || apt-get -y install -f; } || return 1; } ; \
 {% for deb in docker_syncd_mlnx_rpc_pydebs.split(' ') -%}
 dpkg_apt debs/{{ deb }}{{'; '}}
+{%- endfor %}
 
 ## Pre-install the fundamental packages
 RUN apt-get update \