libxmlb: upgrade 0.3.14 -> 0.3.15

libxmlb: upgrade 0.3.14 -> 0.3.15

Changelog:
============
- Sprinkle __attribute__((nonnull)) to give a little more compile-time safety
- Accept text/xml as an alternative to application/xml
- Do not inline shared code
- Fix compiling with Visual Studio
- Fix the exported api test on Windows
- Generate and use .def file for clang-cl builds
- Release source file handles early

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>