add gcc-c++ in dockerfile.gpu

tools/Dockerfile.gpu

@@ -3,7 +3,7 @@ FROM nvidia/cuda:9.0-cudnn7-devel-centos7 as builder
 FROM nvidia/cuda:9.0-cudnn7-runtime-centos7
 RUN yum -y install wget && \
     yum -y install epel-release && yum -y install patchelf && \
-    yum -y install gcc make python-devel && \
+    yum -y install gcc gcc-c++ make python-devel && \
     yum -y install libSM-1.2.2-2.el7.x86_64 --setopt=protected_multilib=false && \
     yum -y install libXrender-0.9.10-1.el7.x86_64 --setopt=protected_multilib=false && \
     yum -y install libXext-1.3.3-3.el7.x86_64 --setopt=protected_multilib=false && \