gtypes: Fix signedness of __builtin_bswap() usage
gtypes: Fix signedness of __builtin_bswap() usage

In GUINT64_TO_BE(), for example, when compiling with -Wsign-conversion,
we get a warning due to an implicit cast from (gint64) to (guint64) when
passing the argument to __builtin_bswap64().

According to the GCC documentation, __builtin_bswap64() takes an
unsigned argument:

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

Cast the input appropriately.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=788384

endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib