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/irc.scm

@@ -136,7 +136,7 @@
            qtbase-5
            qtmultimedia
            qtscript
-           qtsvg
+           qtsvg-5
            snorenotify
            sonnet
            zlib))