checkpatch: Ignore warnings about SPDX

The Linux kernel expects the SPDX license tag to be in the first line of
all source code files. The reason is that they have many different
license headers, and this simplifies the work for any tool that has to
determine the license of each file.

In this project, the license headers follow the same pattern, so it
isn't useful.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

.checkpatch.conf

@@ -16,6 +16,9 @@
 # Show file line, not input line
 --showfile
 
+# Don't expect SPDX tag in the first line of a file
+--ignore SPDX_LICENSE_TAG
+
 # List of ignored rules
 # ---------------------