sl_SI locale: Use "." as the thousands separator (bug 25233)

This is correct according to CLDR [1] and Florian Weimer's quick
research. [2]

[1] https://st.unicode.org/cldr-apps/v#/sl/Symbols/
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=25233#c0



Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>