dev-lang/xsb: add link to upstream bug

Add link to upstream bug for the change in c3d9e65b97.

Also, as a correction, I mentioned a typo Eli fixed in c3d9e65b97,
but that wasn't right (I thought there was an extra _ too).

Bug: https://bugs.gentoo.org/870970
Signed-off-by: Sam James <sam@gentoo.org>

dev-lang/xsb/files/xsb-4.0.0-ld-typo.patch

@@ -1,5 +1,7 @@
 -Wl,-export-dynamic tells the linker we want an entry point of 'xport-dynamic'.
 Fix the typo accordingly.
+
+https://sourceforge.net/p/xsb/bugs/266/
 --- a/build/configure.in
 +++ b/build/configure.in
 @@ -1343,10 +1343,10 @@ else