Workaround for Ruby::BlockTypeMismatch

```
lib/ruby_header_parser/util.rb:20:50: [warning] Cannot pass a value of type `::Proc` as a block-pass-argument of type `^((::String | ::Array[::String])) -> U(1)`
│   ::Proc <: ^((::String | ::Array[::String])) -> U(1)
│
│ Diagnostic ID: Ruby::BlockTypeMismatch
│
└       signature.scan(/[^,]+\([^()]*\)|[^,]+/).map(&:strip)
                                                    ~~~~~~~

```

Author: sue445

lib/ruby_header_parser/util.rb

@@ -17,7 +17,7 @@ def self.find_field(array, field_name)
     # @param signature [String]
     # @return [Array<String>]
     def self.split_signature(signature)
-      signature.scan(/[^,]+\([^()]*\)|[^,]+/).map(&:strip)
+      signature.scan(/[^,]+\([^()]*\)|[^,]+/).flatten.map(&:strip)
     end
 
     # @param type [String]