Log Message:
Bump BUILDLINK_DEPENDS.expat from 1.95.4 to 1.95.7.
The expat-1.95.6nb1 package has libexpat.so.4.0
and expat-1.95.7 has libexpat.so.5.0.

I noticed this because I had a fontconfig package built against
libexpat.so.4 which installed fine on another box
with expat-1.95.7 (libexpat.so.5). The fontconfig package
should have required the expat as now available in pkgsrc.

Please note that I have not bumped PKGREVISIONs for all
the packages depending on this.