Don't add repeated xlmns:xlink declarations in svg-create

* lisp/svg.el (svg-create): Fix previous unconditional addition of
the xmlns:xlink declaration -- callers may already add one, and
having it twice is something most svg libraries doesn't like.
1 job for master in 59 minutes and 27 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #5329
test-all

00:59:27

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1