!134 Serving, fix grpc compiler error

From: @xu-yfei
Reviewed-by: @zhangyinxia,@zhoufeng54
Signed-off-by: @zhangyinxia

cmake/external_libs/grpc.cmake

@@ -1,13 +1,13 @@
 set(grpc_USE_STATIC_LIBS ON)
 if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-    set(grpc_CXXFLAGS "-fstack-protector-all -Wno-uninitialized -Wno-unused-parameter -fPIC -fvisibility=hidden "
-            "-D_FORTIFY_SOURCE=2 -O2")
+    set(grpc_CXXFLAGS "-fstack-protector-all -Wno-uninitialized -Wno-unused-parameter -fPIC -fvisibility=hidden \
+            -D_FORTIFY_SOURCE=2 -O2")
 elseif(${CMAKE_SYSTEM_NAME} MATCHES "Windows")
-    set(grpc_CXXFLAGS "-fstack-protector-all -Wno-maybe-uninitialized -Wno-unused-parameter -fPIC -fvisibility=hidden "
-            "-D_FORTIFY_SOURCE=2 -O2")
+    set(grpc_CXXFLAGS "-fstack-protector-all -Wno-maybe-uninitialized -Wno-unused-parameter -fPIC -fvisibility=hidden \
+            -D_FORTIFY_SOURCE=2 -O2")
 else()
-    set(grpc_CXXFLAGS "-fstack-protector-all -Wno-maybe-uninitialized -Wno-unused-parameter -fPIC -fvisibility=hidden "
-            "-D_FORTIFY_SOURCE=2 -D_GLIBCXX_USE_CXX11_ABI=0 -O2")
+    set(grpc_CXXFLAGS "-fstack-protector-all -Wno-maybe-uninitialized -Wno-unused-parameter -fPIC -fvisibility=hidden \
+            -D_FORTIFY_SOURCE=2 -D_GLIBCXX_USE_CXX11_ABI=0 -O2")
 endif()
 
 set(grpc_LDFLAGS "-Wl,-z,relro,-z,now,-z,noexecstack")