main/ncurses: upgrade to 6.2_p20200509

change source url because mirror doesn't work now,
from the mailing list announce:
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Patches for ncurses 6.2 can be found at
       ftp://ftp.invisible-island.net/ncurses/6.2

The hosting provider for this site made some "update" which breaks my
process for mirroring (I'll resume mirroring when I'm able):
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8 jobs for !7792 with ncurses-6.2-p20200509 in 3 minutes and 29 seconds
latest detached
Status Job ID Name Coverage
  Lint
passed #116732
x86_64 docker-alpine
lint

00:00:29

 
  Build
passed #116737
aarch64 ci-build docker-alpine
build-aarch64

00:01:29

passed #116738
armv7 ci-build docker-alpine
build-armv7

00:01:49

passed #116736
ppc64le ci-build docker-alpine
build-ppc64le

00:01:52

passed #116783
docker-alpine ci-build s390x
build-s390x

00:01:04

passed #116734
x86 ci-build docker-alpine
build-x86

00:01:05

passed #116733
x86_64 ci-build docker-alpine
build-x86_64

00:01:11

failed #116735
s390x ci-build docker-alpine
build-s390x

02:00:01