ParserGen: Support generating bindings on Linux/macOS

The previous approach (using backslashes for path separators) only works
on Windows.

src/CppParser/ParserGen/ParserGen.cs

@@ -221,7 +221,7 @@ namespace CppSharp
                 Console.WriteLine();
             }
 
-            var osxHeadersPath = Path.Combine(GetSourceDirectory("build"), @"headers\osx");
+            var osxHeadersPath = Path.Combine(GetSourceDirectory("build"), "headers", "osx");
             if (Directory.Exists(osxHeadersPath) || Platform.IsMacOS)
             {
                 Console.WriteLine("Generating the C# parser bindings for OSX x86...");
@@ -237,7 +237,7 @@ namespace CppSharp
                 Console.WriteLine();
             }
 
-            var linuxHeadersPath = Path.Combine(GetSourceDirectory("build"), @"headers\x86_64-linux-gnu");
+            var linuxHeadersPath = Path.Combine(GetSourceDirectory("build"), "headers", "x86_64-linux-gnu");
             if (Directory.Exists(linuxHeadersPath) || Platform.IsLinux)
             {
                 Console.WriteLine("Generating the C# parser bindings for Linux...");