fix path normalization for linux system(MessagePack-CSharp#386)

sandbox/TestData.SubDir/Class1.cs

@@ -0,0 +1,12 @@
+using System;
+using MessagePack;
+
+namespace TestData.SubDir
+{
+
+    [MessagePackObject(true)]
+    public class A
+    {
+        public int X { get; set; }
+    }
+}

sandbox/TestData.SubDir/Sub/SubA.cs

@@ -0,0 +1,12 @@
+using System;
+using MessagePack;
+
+namespace TestData.SubDir.Sub
+{
+
+    [MessagePackObject(true)]
+    public class SubA
+    {
+        public int Y { get; set; }
+    }
+}

sandbox/TestData.SubDir/TestData.SubDir.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="MessagePack" Version="1.7.3.4" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Remove="Sub\*.cs"/>
+    <Compile Include="Sub\SubA.cs"/>
+  </ItemGroup>
+
+</Project>

src/MessagePack.UniversalCodeGenerator/Utils/RoslynExtensions.cs

@@ -216,7 +216,8 @@ static Workspace GetWorkspaceFromBuild(this StLogger.Build build, params string[
             foreach (var compile in compileItems)
             {
                 var filePath = compile.Text;
-                var absFilePath = Path.Combine(projectDir, filePath);
+                // normalize path separater char
+                var absFilePath = Path.Combine(projectDir, filePath).Replace('\\', Path.DirectorySeparatorChar);
                 roslynProject = roslynProject.AddDocument(filePath, File.ReadAllText(absFilePath)).Project;
             }
             foreach (var asm in assemblies)