www/qt5-webengine: brand installed shared libraries

For some reason, the shared libraries installed by this package are
branded as being for the SYSV ABI instead of the FreeBSD ABI.  This
confuses pkg, making dependent packages fail in build-depends.
Stick a FreeBSD ELF brand on to work around this issue.

Reported by:	Mark Millard <marklmi@yahoo.com>
MFH:		2025Q2