Include directly dependent header

[372046933]

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• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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bool SupportedByRdma(std::string protocol); use std::string but forgets to #include <string>
See https://github.com/apache/incubator-brpc/blob/7998e68b89164d806933a8deddaa0b8b8f125f1b/src/brpc/rdma/rdma_helper.h#L77

Compile error like this can happen

external/brpc/src/brpc/rdma/rdma_helper.h:77:27: error: ‘string’ is not a member of ‘std’
   77 | bool SupportedByRdma(std::string protocol);
      |                           ^~~~~~
external/brpc/src/brpc/rdma/rdma_helper.h:24:1: note: ‘std::string’ is defined in header ‘<string>’; did you forget to ‘#include <string>’?
   23 | #include <infiniband/verbs.h>
  +++ |+#include <string>
   24 |