require java.xml directly so that java.desktop must not be available

[tomsontom]

fixes #490

[tomsontom]

As outlined in #490 the question is whether it should be require static and the code in DefaultSerializers should gracefully handle the CNF and allow users to even get rid of java.xml dependency if they don't need it.

From a backwards compat point of view there's no difference. Nobody could have used yasson today unless they required java.desktop anyways so adding the java.xml dependency does not alter their runtime image.

[mkarg]

@m0mus Dmitry, kindly pinging you. :-)

@tomsontom Please fix the build fail, see failing checks above (wrong copyright year). Thanks.

[tomsontom]

@Verdent I think I implemented what you suggested in #490
@mkarg I fixed the build issue interesting although is that the checkstyle:checkstyle goal fails locally in a class I haven touched (StrategiesProvider.java) but succeeds on travis

[Verdent]

LGTM. Thank you :-)