[contrib][linux] Add contrib/linux-kernel/linux to .gitignore

The mentioned path is being created/used by the 'import' rule for
generating source files for Linux kernel.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>

.gitignore

@@ -29,6 +29,7 @@ dictionary
 NUL
 
 # Build artefacts
+contrib/linux-kernel/linux/
 projects/
 bin/
 .buckd/