x11-fonts/fontconfig: add upstream fix 49-sansserif.conf

After updating fontconfig 2.13.93, the configuration file
'49-sansserif.conf' shipped with fontconfig itself stops working. This
means if some glyphs cannot be found in the given font, fontconfig will
pick fallback fonts randomly instead of honoring what the user sets as
sans-serif.

This commit pulls in upstreams fix for this issue from
https://gitlab.freedesktop.org/fontconfig/fontconfig/-/merge_requests/142

See also upstreams PR:
https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/267

PR: 256969
(cherry picked from commit 4b4126ceba9b8adf8d7b6a6af421923ebba86118)