sysutils/screen: Bump PORTREVISION to chase src 21817992b331

21817992b331 in FreeBSD/main installed ncurses 6.5 into 15-CURRENT.
This causes screen to display errors about duplicate entries in
termcap/terminfo. A rebuild of screen fixes the problem.

The errors are:

Name collision 'cdc456tst' between

cdc456tst

and cdc456tst
Cannot remove alias 'cdc456tst'
Name collision 'tab132w' between

tab132w

and tab132w
Cannot remove alias 'tab132w'

And many more errors like the above.