fix build

[pengyongqiang666]

fix below build error

INFO: Analyzed target //:envoy (0 packages loaded, 0 targets configured).
ERROR: /Users/pyq/CLionProjects/envoy-filter-example/BUILD:33:17: Compiling echo2_config.cc failed: (Exit 1)
: 
cc_wrapper.sh failed: error executing CppCompile command (from target //:echo2_config) external/local_config
_cc/cc_wrapper.sh -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -Wunused-but-set-p
arameter -Wno-free-nonheap-object -fcolor-diagnostics ... (remaining 190 arguments skipped)
      
Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
echo2_config.cc:17:28: error: virtual function 'createFilterFactoryFromProto' has a different return type ('
Ne
twork::FilterFactoryCb' (aka 'function<void (FilterManager &)>')) than the function it overrides (which has
return type 'absl::StatusOr<Network::FilterFactoryCb>' (aka 'StatusOr<function<void (FilterManager &)>>'))
       17 |   Network::FilterFactoryCb createFilterFactoryFromProto(const Protobuf::Message&,
      |   ~~~~~~~~~~~~~~~~~~~~~~~~ ^
external/envoy/envoy/server/filter_config.h:177:3: note: overridden virtual function is here
  176 |   virtual absl::StatusOr<Network::FilterFactoryCb>
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  177 |   createFilterFactoryFromProto(const Protobuf::Message& config,
      |   ^
1 error generated.
Target //:envoy failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 27.027s, Critical Path: 18.67s
INFO: 15 processes: 12 internal, 3 darwin-sandbox.
ERROR: Build did NOT complete successfully