Include sys/stat.h to define S_* properly

Without including sys/stat.h, this file failed to build on FreeBSD with the following error.

In file included from /var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/test_trueaudio.cpp:5:0:
/var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/utils.h: In function 'std::string copyFile(const string&, const string&)':
/var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/utils.h:36:62: error: 'S_IRUSR' was not declared in this scope
/var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/utils.h:36:72: error: 'S_IWUSR' was not declared in this scope
In file included from /var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/test_mpeg.cpp:6:0:
/var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/utils.h: In function 'std::string copyFile(const string&, const string&)':
/var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/utils.h:36:62: error: 'S_IRUSR' was not declared in this scope
/var/tmp/portage/media-libs/taglib-1.8/work/taglib-1.8/tests/utils.h:36:72: error: 'S_IWUSR' was not declared in this scope
gmake[2]: *** [tests/CMakeFiles/test_runner.dir/test_mpeg.cpp.o] Error 1