www/webkit2-gtk3: fix build with clang 16

Clang 16 has a new error about integer values being outside the valid
range for enum types, which shows up when building www/webkit2-gtk3:

/wrkdirs/usr/ports/www/webkit2-gtk3/work/webkitgtk-2.34.6/Source/WTF/wtf/URLHelpers.cpp:51:34: error: integer value 256 is outside the valid range of values [-256, 255] for this enumeration type [-Wenum-constexpr-conversion]
constexpr auto scriptCodeLimit = static_cast<UScriptCode>(256);
                                 ^

Upstream fixed this in https://commits.webkit.org/256560@main, for
https://bugs.webkit.org/show_bug.cgi?id=247770, so apply that patch.
Thanks to Vladislav Shchapov for pointing out the upstream fix.

PR: 271827
Approved by: portmgr (build blanket)
MFH: 2023Q2