Compiler error on GCC 9.0+

[fboemer]

On machine with GCC 9.0+, python3 build_ngtf.py yields

**ERROR: /localdisk/fboemer/.cache/bazel/_bazel_fboemer/3c1c0ab49f097ff69f68c8568101c966/external/grpc/BUILD:507:1: C++ compilation of rule '@grpc//:gpr_base' failed (Exit 1)
external/grpc/src/core/lib/gpr/log_linux.cc: In function 'long int gettid()':
external/grpc/src/core/lib/gpr/log_linux.cc:43:13: error: ambiguating new declaration of 'long int gettid()'
 static long gettid(void) { return syscall(__NR_gettid); }
             ^~~~~~
In file included from /usr/include/unistd.h:1170:0,
                 from external/grpc/src/core/lib/gpr/log_linux.cc:41:
/usr/include/x86_64-linux-gnu/bits/unistd_ext.h:34:16: note: old declaration '__pid_t gettid()'
 extern __pid_t gettid (void) __THROW;
                ^~~~~~
external/grpc/src/core/lib/gpr/log_linux.cc: At global scope:
external/grpc/src/core/lib/gpr/log_linux.cc:43:13: warning: 'long int gettid()' defined but not used [-Wunused-function]
 static long gettid(void) { return syscall(__NR_gettid); }
             ^~~~~~
Target //tensorflow/tools/pip_package:build_pip_package failed to build**

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