makepot.sh: Specify source character encoding as UTF-8

[ci skip]

Author: tobbi

makepot.sh

@@ -5,7 +5,7 @@ package_version="$(git describe --tags --match "?[0-9]*.[0-9]*.[0-9]*")"
 
 xgettext --keyword='_' --keyword='__:1,2' -C -o data/locale/main.pot \
   $(find src -name "*.cpp" -or -name "*.hpp") \
-  --add-comments=l10n \
+  --from-code=UTF-8 --add-comments=l10n \
   --package-name="${package_name}" --package-version="${package_version}" \
   --msgid-bugs-address=https://github.com/SuperTux/supertux/issues