svgtiny: update source url

As source url use the git repository instead of the unavaiable svn
repository.

This fix the following error:

svn: E670002: Unable to connect to a repository at URL 'svn://svn.netsurf-browser.org/trunk/libsvgtiny

'
svn: E670002: Unknown hostname 'svn.netsurf-browser.org'

[Thomas: use http:// access to the Git repository, as suggested by
Baruch.]

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>