Fix FrozenError when parsing ERB files in Ruby 4.0 (#118)

Running `rake gettext:find` on Ruby 4.0 projects fails with the
following error:
```
FrozenError: can't modify frozen String: "#coding:UTF-8\n_erbout = +''; _erbout.<< "<h2>".freeze;
```
In Ruby 4.0, `ERB#src` now returns a frozen String.
`GetText::ErbParser#parse` calls src.force_encoding(encoding) directly
on that value, which raises a FrozenError.

To fix that, we're using the unary + operator (`+erb.src`) to get a
mutable copy of the string.

Author: karinevieira

lib/gettext/tools/parser/erb.rb

@@ -70,7 +70,7 @@ def parse
       else
         erb = ERB.new(content, nil, "-")
       end
-      src = erb.src
+      src = +erb.src
 
       # Force the src encoding back to the encoding in magic comment
       # or original content.