package/postgresql: bump version to 11.0

Release notes: https://www.postgresql.org/about/news/1894/

Added md5 hash provided by upstream.

To fix build errors "MAKELEVEL=0" is required by upstream in MAKE_OPTS:
https://www.postgresql-archive.org/BUG-15407-minor-build-depends-on-MAKELEVEL-being-0-at-top-Makefile-td6047685.html#a6047709



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: add a comment in the .mk file about MAKELEVEL=0]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>