tools: allow // SPDX... in checkpatch.pl

Using // style comments for the SPDX license identifier was kind of an
intentional choice to make it stand out as "directive-like" comment (and
also to constrain it to the one line.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

tools/checkpatch.pl

@@ -3627,7 +3627,9 @@ sub process {
 
 # no C99 // comments
 		if ($line =~ m{//}) {
-			if (!$allow_c99_comments) {
+			if ($rawlines[$linenr - 1] =~ /SPDX-License-Identifier:/) {
+				# ignore
+			} elsif (!$allow_c99_comments) {
 				if(ERROR("C99_COMMENTS",
 					 "do not use C99 // comments\n" . $herecurr) &&
 				   $fix) {