.gitignore: adjust paths for autogenerated plural_formula sources

Ref: https://github.com/openwrt/luci/pull/5976#issuecomment-1290370037
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

.gitignore

@@ -10,8 +10,8 @@ package-lock.json
 modules/luci-base/src/po2lmo
 modules/luci-base/src/jsmin
 modules/luci-base/src/contrib/lemon
-modules/luci-base/src/ucode/plural_formula.c
-modules/luci-base/src/ucode/plural_formula.h
+modules/luci-base/src/lib/plural_formula.c
+modules/luci-base/src/lib/plural_formula.h
 modules/luci-compat/src/contrib/lemon
 modules/luci-compat/src/plural_formula.c
 modules/luci-compat/src/plural_formula.h