github: Mark some more files as derived / generated

Since we started committing generated files as well we can mark them as
derived so that they appear collapsed in the diff, reducing noise while
reviewing.

.gitattributes

@@ -7,4 +7,8 @@ configure text eol=lf
 # The following files are generated and should not be shown in the
 # diffs by default on Github.
 doc/lightning*.7 linguist-generated=true
+wallet/db_*_sqlgen.c linguist-generated=true
+wallet/statements_gettextgen.po linguist-generated=true
+*_wiregen.? linguist-generated=true
+*_printgen.? linguist-generated=true