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.

sysutils/screen/Makefile

@@ -1,6 +1,6 @@
 PORTNAME=	screen
 PORTVERSION=	4.9.1
-PORTREVISION=	2
+PORTREVISION=	3
 CATEGORIES=	sysutils
 MASTER_SITES=	GNU \
 		ftp://ftp.gnu.org/gnu/screen/ \