Fix gemspec: correct VC project glob and exclude generated extconf.h

The VC project glob had the wrong extension (.vcprojx vs .vcxproj) and
wrong directory depth, so the .vcxproj was never packaged. Also exclude
the generated extconf.h so local build config doesn't leak into the gem.

Author: cfis

libxml-ruby.gemspec

@@ -29,11 +29,12 @@ Gem::Specification.new do |spec|
                          'ext/libxml/*.h',
                          'ext/libxml/*.c',
                          'ext/libxml/*.rb',
-                         'ext/vc/*.sln',
-                         'ext/vc/*.vcprojx',
+                         'ext/vc/**/*.sln',
+                         'ext/vc/**/*.vcxproj',
                          'ext/xcode/**/*',
                          'lib/**/*.rb',
-                         'test/**/*'])
+                         'test/**/*']) -
+                Dir.glob(['ext/libxml/extconf.h'])
 
   spec.test_files = Dir.glob('test/test_*.rb')
   spec.required_ruby_version = '>= 3.2'