Added .lib extension for library file on VS2012

tasks/toolchains/vs2012.rake

@@ -14,7 +14,7 @@ MRuby::Toolchain.new(:vs2012) do |conf|
     linker.flags = [ENV['LDFLAGS'] || %w(/nologo)]
     linker.libraries = %w()
     linker.library_paths = %w()
-    linker.option_library = '%s'
+    linker.option_library = '%s.lib'
     linker.option_library_path = '/LIBPATH:%s'
     linker.link_options = "%{flags} /OUT:%{outfile} %{objs} %{flags_before_libraries} %{libs} %{flags_after_libraries}"
   end