[PostGIS] #5968: Remove generated files in distclean target

[PostGIS]

#5968: Remove generated files in distclean target
----------------------------+---------------------------
 Reporter:  Bas Couwenberg  |      Owner:  strk
     Type:  defect          |     Status:  new
 Priority:  medium          |  Milestone:  PostGIS 3.6.0
Component:  build           |    Version:  master
 Keywords:                  |
----------------------------+---------------------------
 The Debian package failed to build twice in a row because generated files
 were not removed in the clean target:
 {{{
 dpkg-source: info: local changes detected, the modified files are:
  source_dir/raster/test/regress/tests.mk
 }}}

 The attached patch fixes the issue
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/5968>
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