Bug#952531: bump build dependencies to gnat-9

[Matthias Klose]

Package: src:plplot
Version: 5.15.0+dfsg-10+gnat8+1
Severity: serious
Tags: sid bullseye patch

plplot's build dependencies fail to install after the start of the gnat-9
transition.  Please build-depend on gnat-9 instead of gnat-8.