fix -DSERVER problem

tools/serving_build.sh

@@ -82,7 +82,7 @@ function build_server() {
             cmake -DPYTHON_INCLUDE_DIR=$PYTHONROOT/include/python2.7/ \
                   -DPYTHON_LIBRARIES=$PYTHONROOT/lib64/libpython2.7.so \
                   -DPYTHON_EXECUTABLE=$PYTHONROOT/bin/python \
-                  -DSERVER=OFF ..
+                  -DSERVER=ON ..
             rerun "make -j2 >/dev/null" 3 # due to some network reasons, compilation may fail
             check_cmd "make install -j2 >/dev/null"
             pip install -U python/dist/paddle_serving_server* >/dev/null