graphics/gdal needs a PORTREVISION bump

From: Philipp Ost <pj_at_smo.de>
Date: Sat, 17 Jul 2021 18:35:56 UTC
Hi ports@,

graphics/gdal seems to have fallen through the cracks of the recent 
Imath update. It is not included in the list of ports in commit 
3cda93bdf69bfb4ea99181e638e661c467dad38d.

Compiling blender fails with the following error:
ld-elf.so.1: Shared object "libImath-3_0.so.28" not found, required by 
"libgdal.so.3"

Rebuilding gdal to pick up the new libImath fixed that.

Philipp