graphics/openimageio: fix build on powerpc64 elfv2

Change if to explicit ifdef to prevent:
/wrkdirs/usr/ports/graphics/openimageio/work/oiio-Release-2.0.10/src/libutil/ustring.cpp:328:27: error: expected value in expression

if _LIBCPP_BIG_ENDIAN

PR: 242134
Approved by: FreeBSD@ShaneWare.Biz (maintainer timeout)