build: Stop building gnu/build/svg.scm.

* Makefile.am (EXTRA_DIST): Add gnu/build/svg.scm.
* gnu/local.mk (GNU_SYSTEM_MODULES): Remove gnu/build/svg.scm.

It fixes the following build warnings:

gnu/build/svg.scm:36:12: warning: possibly unbound variable `cairo-create'
gnu/build/svg.scm:36:26: warning: possibly unbound variable `cairo-image-surface-create'
gnu/build/svg.scm:38:4: warning: possibly unbound variable `cairo-scale'
gnu/build/svg.scm:39:4: warning: possibly unbound variable `cairo-set-source-surface'

Signed-off-by: Ludovic Courtès <ludo@gnu.org>