Fix -Wint-conversion error with gcc 14 due to musl using POSIX strerror_r.

```
/home/build/aports/testing/fluent-bit/src/fluent-bit-3.1.9/src/flb_network.c: In function 'net_connect_async':
/home/build/aports/testing/fluent-bit/src/fluent-bit-3.1.9/src/flb_network.c:566:17: error: assignment to 'char *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
  566 |             str = strerror_r(error, so_error_buf, sizeof(so_error_buf));
```

--- fluent-bit-3.1.9-origin/src/flb_network.c
+++ fluent-bit-3.1.9/src/flb_network.c
@@ -549,24 +549,18 @@
             if (u_conn->net_error > 0) {
                 error = u_conn->net_error;
             }
 
             /* Connection is broken, not much to do here */
-#if ((defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) ||    \
-     (defined(_XOPEN_SOURCE) || _XOPEN_SOURCE - 0L >= 600L)) &&     \
-  (!defined(_GNU_SOURCE))
             ret = strerror_r(error, so_error_buf, sizeof(so_error_buf));
             if (ret == 0) {
                 str = so_error_buf;
             }
             else {
                 flb_errno();
                 return -1;
             }
-#else
-            str = strerror_r(error, so_error_buf, sizeof(so_error_buf));
-#endif
             flb_error("[net] TCP connection failed: %s:%i (%s)",
                       u->tcp_host, u->tcp_port, str);
             return -1;
         }
     }