Cleanup: move declaration of get_stylesheet() to qthelper.h

The function is defined in qthelper.c and thus not all users
of dive.h have to suck in the xslt headers.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/uemis-downloader.c

@@ -16,6 +16,7 @@
 #include <fcntl.h>
 #include <dirent.h>
 #include <stdio.h>
+#include <stdarg.h>
 #include <unistd.h>
 #include <string.h>
 #include <errno.h>