utfcpp: upgrade 4.0.6 -> 4.0.8

utfcpp: upgrade 4.0.6 -> 4.0.8

License-update: the project removed the submodules, and included
the necessary files in the main repository. The license itself hasn't
chaged, however it seems that it was indicated incorrectly in the recipe
previously: the main application is BSL-1.0, but the additional header
is under MIT, and not BSD.

Selection from the shortlog:
- fix compile warning
- Get rid of submodules and add ftest.h to tests directry
- tidy code
- Compile time checks for size of UTF-16 code units

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>