Workaround for [error] cannot find compatible overloading of method system of type ::RubyHeaderParser::Parser

```
lib/ruby_header_parser/parser.rb:150:8: [error] Cannot find compatible overloading of method `system` of type `::RubyHeaderParser::Parser`
│ Method types:
│   def system: (::String, *::String, ?unsetenv_others: ::boolish, ?pgroup: (true | ::Integer), ?umask: ::Integer, ?in: ::Kernel::redirect_fd, ?out: ::Kernel::redirect_fd, ?err: ::Kernel::redirect_fd, ?close_others: ::boolish, ?chdir: ::String) -> (::NilClass | ::FalseClass | ::TrueClass)
│             | (::Hash[::string, (::string | nil)], ::String, *::String, ?unsetenv_others: ::boolish, ?pgroup: (true | ::Integer), ?umask: ::Integer, ?in: ::Kernel::redirect_fd, ?out: ::Kernel::redirect_fd, ?err: ::Kernel::redirect_fd, ?close_others: ::boolish, ?chdir: ::String) -> (::NilClass | ::FalseClass | ::TrueClass)
│
│ Diagnostic ID: Ruby::UnresolvedOverloading
│
└         system("gcc -E #{include_args} #{header_file} -o #{dist_preprocessed_header_file}", exception: true)
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Author: sue445

ruby_header_parser.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
   spec.files = IO.popen(%w[git ls-files -z], chdir: __dir__, err: IO::NULL) do |ls|
     ls.readlines("\x0", chomp: true).reject do |f|
       (f == gemspec) ||
-        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile])
+        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile sig/non-gemify])
     end
   end
   spec.bindir = "exe"

sig/non-gemify/kernel.rbs

@@ -0,0 +1,6 @@
+module Kernel
+  # FIXME: monkey patch for `exception` arg of `Kernel.system`
+  def self?.system: (String command, *String args, ?unsetenv_others: boolish, ?pgroup: true | Integer, ?umask: Integer, ?in: redirect_fd, ?out: redirect_fd, ?err: redirect_fd, ?close_others: boolish, ?chdir: String, ?exception: bool) -> (NilClass | FalseClass | TrueClass)
+    | (Hash[string, string?] env, String command, *String args, ?unsetenv_others: boolish, ?pgroup: true | Integer, ?umask: Integer, ?in: redirect_fd, ?out: redirect_fd, ?err: redirect_fd, ?close_others: boolish, ?chdir: String, ?exception: bool) -> (NilClass | FalseClass | TrueClass)
+    | ...
+end