Fix gettid() naming conflict

src/core/lib/gpr/log_linux.cc

@@ -40,7 +40,9 @@
 #include <time.h>
 #include <unistd.h>
 
-static long gettid(void) { return syscall(__NR_gettid); }
+// Not naming it as gettid() to avoid duplicate declarations when complied with
+// GCC 9.1.
+static long local_gettid(void) { return syscall(__NR_gettid); }
 
 void gpr_log(const char* file, int line, gpr_log_severity severity,
              const char* format, ...) {
@@ -70,7 +72,7 @@ void gpr_default_log(gpr_log_func_args* args) {
   gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
   static __thread long tid = 0;
-  if (tid == 0) tid = gettid();
+  if (tid == 0) tid = local_gettid();
 
   timer = static_cast<time_t>(now.tv_sec);
   final_slash = strrchr(args->file, '/');