slirp: clarify license of slirp files using SPDX: implicit via COPYRIGHT

Add SPDX license identifier to clarify the license of files with
reference to BSD license from slirp COPYRIGHT file.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

slirp/src/debug.h

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/if.c

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/if.h

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/main.h

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/mbuf.c

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski
  *

slirp/src/misc.c

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/misc.h

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/sbuf.c

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/sbuf.h

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/socket.c

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *

slirp/src/socket.h

+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1995 Danny Gasparovski.
  *