gnu: qtsvg: Rename variable to qtsvg-5.

This is in preparation of the qtsvg update to version 6.

This change was automated via the following command:

  git grep -l '\bqtsvg\b' | xargs sed 's/qtsvg/qtsvg-5/g' -i

gnu/packages/image-processing.scm

@@ -1231,7 +1231,7 @@ substituted by matching images.")
          ("libpng" ,libpng)
          ("libtiff" ,libtiff)
          ("qtbase" ,qtbase-5)
-         ("qtsvg" ,qtsvg)
+         ("qtsvg-5" ,qtsvg-5)
          ("zlib" ,zlib)))
       (arguments
        `(#:phases