component/libjpeg: Fix compilation (md5sum mismatch)

It's seems that the maintainer of https://www.ijg.org changed the source
files of libjpeg (even without changing the timestamp) [1]. We could
either update the md5sum or change the source link. Since this may
happen again it seems to be less maintenance efforts to change the
source to a more reliable, less frequently changing source (unless we
explicitly want the update). Because we also use buildout.net in other
components [2], this looks like an acceptable replacement.

[1] https://github.com/conan-io/conan-center-index/issues/4151
[2] https://lab.nexedi.com/nexedi/slapos/blob/a86050ed/component/libcap-ng/buildout.cfg#L9

component/libjpeg/buildout.cfg

@@ -9,7 +9,7 @@ parts =
 [libjpeg]
 recipe = slapos.recipe.cmmi
 shared = true
-url = http://www.ijg.org/files/jpegsrc.v9d.tar.gz
+url = http://sources.buildroot.net/libjpeg/jpegsrc.v9d.tar.gz
 md5sum = 693a4e10906e66467ca21f045547fe15
 configure-options =
   --disable-static