libsvgtiny: update source url

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

Also because the git repository does not include the netsurf core
buildsystem source that are needed to build this package add as
dependency the netsurf-buildsystem package and use those files through a
symbolink link.

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'

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