kernel: modules: Add SPDX license identifier to kmod.c

Add a GPL-2.0 license identifier line for this file.

kmod.c was originally introduced in the kernel in February
of 1998 by Linus Torvalds - who was familiar with kernel
licensing at the time this was introduced.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/module/kmod.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * kmod - the kernel module loader
  *