d/s/lintian-overrides: add license-problem-non-free-RFC error in inet-imap gem

This seems to be a false positive. There is no RFC text in that gem.

Author: Lucas Kanashiro

debian/source/lintian-overrides

@@ -1 +1,4 @@
 source-contains-prebuilt-ms-help-file
+# There is no RFC text
+license-problem-non-free-RFC .bundle/gems/net-imap-0.3.4/LICENSE.txt
+license-problem-non-free-RFC-BCP78 .bundle/gems/net-imap-0.3.4/LICENSE.txt