docs/manual/prerequisite.txt: 'make xconfig' uses Qt5, not Qt4
Since commit 6eacea5a (support/kconfig: bump to kconfig from Linux
4.17-rc2), xconfig uses Qt5, so update prerequisite.txt to match.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>