x11-toolkits/gtk40: un-conditionalise bswap64()/bswap_64()

All currently supported base system versions/branches include the
glibc-compatible <byteswap.h> containing bswap_64(), which maps
directly to <sys/_endian.h> __bswap64(). Consolidation in GTK's
roaring component resulted in an extra preprocessor guard for us.
This fixes possible build errors when <sys/endian.h> is not included
by other means.

Upstream commit: https://gitlab.gnome.org/GNOME/gtk/-/commit/047c8e64d4156897776ec2bc3d0613a81cfe7ad1