Use SPDX header for license

Use SPDX header for license

Drop the lengthy license from each file and just use SPDX like most
projects nowadays. This doesn't have any change to license, just how
they are recorded in each file.

This follows the kernel approach: header files use '/*' for comments
while .c files use '//'. For .m4, use "#".

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20240723185921.1005569-2-lucas.de.marchi@gmail.com
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>