Ensure proper encoding in non-UTF-8 locales.

https://bugs.gentoo.org/640150

--- a/lib/kpeg/grammar.rb.orig	2018-05-20 09:26:13.597648830 +0200
+++ b/lib/kpeg/grammar.rb	2018-05-20 09:28:19.606460712 +0200
@@ -130,6 +130,10 @@
           end
         end
 
+	if reg =~ /\{[LMNPSZC][c-u]?\}/ && reg.encoding != Encoding::UTF_8
+	  reg.force_encoding('UTF-8')
+	end
+
         @regexp = Regexp.new(reg, flags, lang)
       else
         @regexp = reg