tcp_bbr: fix SPDX-License-Identifier to be GPL-2.0 OR BSD-3-Clause

Since TCP BBR congestion control was introduced in
commit 0f8782ea14 ("tcp_bbr: add BBR congestion control")
it has always been offered as "Dual BSD/GPL":

  MODULE_LICENSE("Dual BSD/GPL");

A GPL-2.0-only SPDX header was erroneously added in the recent
commit 2ed4b46b4f ("net: Add SPDX ids to some source files").

This commit revises the tcp_bbr.c SPDX-License-Identifier to note that
this file is licensed as "GPL-2.0 OR BSD-3-Clause".

Fixes: 2ed4b46b4f ("net: Add SPDX ids to some source files")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Van Jacobson <vanj@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Tim Bird <tim.bird@sony.com>
Link: https://patch.msgid.link/20260531183558.2337381-1-ncardwell.sw@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_bbr.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
 /* Bottleneck Bandwidth and RTT (BBR) congestion control
  *
  * BBR congestion control computes the sending rate based on the delivery