Add MSYS2 platform package requirement

## Issue

On MSYS2 environments, installing gems that depend on
pkgconf might fail because the pkgconf system package
cannot be automatically installed.

## Cause

The pkg-config gem was missing MSYS2 platform
configuration in its system package requirements.
Without this, the rubygems-requirements-system gem
cannot automatically install the pkgconfg system
package on MSYS2 environments.

## Solution

Add `system: pkgconf: msys2: pkgconf` to the gemspec
requirements.
This works with rubygems-requirements-system which
supports MSYS2 platform.

Author: otegami

pkg-config.gemspec

@@ -32,5 +32,7 @@ Gem::Specification.new do |spec|
   spec.files += Dir.glob("lib/**/*.rb")
   spec.test_files = Dir.glob("test/**/*.rb")
 
+  spec.requirements << "system: pkgconf: msys2: pkgconf"
+
   spec.metadata["msys2_mingw_dependencies"] = "pkgconf"
 end