sstrip: fix source svn repository url

Because the svn repository url was changed the command "make
sstrip-source" gives this error:

svn: E000101: Unable to connect to a repository at URL 'svn://dev.openwrt.org/openwrt/trunk/tools/sstrip'
svn: E000101: Can't connect to host 'dev.openwrt.org': Network is unreachable

To fix this error use the correct url "svn://svn.openwrt.org/openwrt

".

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>