www/qt5-webkit: Fix fetch of glib 2.68 workaround

The filesize of the glib 2.68 workaround changes for some reason
intermittently from 841 bytes to 839 bytes when it's fetched:

$ diff 5b698ba3faff_841_bytes.patch 5b698ba3faff_839_bytes.patch
14c14

< index 06133d82cb35d..d9a1d2f145f50 100644

index 06133d82cb35..d9a1d2f145f5 100644

Therefore convert the patch into a local patch to remedy the issue.

Reported by: tcberner, yuri
MFH: No (not required, devel/glib20 is at 2.66.8 in 2021Q3)