build: fix linker error with kasumi_test

When the osmocore package is configured with --disable-static,
building of kasumi_test fails. This seems quite legit, given
the function _kasumi_kgcore is not exported.
Don't try to workaround the build system. Include the code.

  CCLD     kasumi/kasumi_test
kasumi_test.o: In function `test_expansion':
~tests/kasumi/kasumi_test.c:25: undefined reference to `_kasumi_key_expand'
kasumi_test.o: In function `main':
~tests/kasumi/kasumi_test.c:56: undefined reference to `_kasumi'
~tests/kasumi/kasumi_test.c:100: undefined reference to `_kasumi_key_expand'
~tests/kasumi/kasumi_test.c:112: undefined reference to `_kasumi_kgcore'
[...]
1 file changed