graphics/vigra: fix build with clang 19

Clang 19 is more strict about manipulating this in C++, which results
in an error compiling graphics/vigra:

/wrkdirs/usr/ports/graphics/vigra/work/vigra-8acd73a/include/vigra/multi_iterator_coupled.hxx:493:9: error: expression is not assignable
  493 |         --this;
      |         ^ ~~~~

Here, the intent is to call the prefix operator-- on the object, which
should be spelled either "--*this" or "operator--()".

PR: 281358
Approved by: maintainer timeout (2 weeks)
MFH: 2024Q3

(cherry picked from commit 5a269b5cd4965710fa4e02fdb1cc2ffe6def6921)