PCH #include scanner: force string encoding to be UTF-8

If the locale is set to non-UTF-8 when (d)rake is invoked then Ruby will
set the encoding of all strings read from files to US-ASCII. As several
source files do use non-ASCII characters this results in warnings from
Ruby about "invalid byte sequence in US-ASCII".

As all of my source code files are encoded in UTF-8 we can simply
enforce this.

This happens when building the RPMs which sets the locale to C.

rake.d/pch.rb

@@ -383,6 +383,7 @@ PCH status: <%= c?(:USE_PRECOMPILED_HEADERS) ? "enabled" : "disabled" %>
     input = indirect ? indirect : user
     File.open(input) do |f|
       f.each_line do |line|
+        line.force_encoding("UTF-8")
         next if !@scan_include_re.match(line)
         @scan_candidates.each do |pair|
           (dir,header) = *pair