Add new file to the portable System.Memory

Author: jkotas

src/System.Memory/src/System.Memory.csproj

@@ -114,6 +114,7 @@
     <Compile Include="$(CommonPath)\CoreLib\System\SpanDebugView.cs" />
     <Compile Include="$(CommonPath)\CoreLib\System\SpanHelpers.BinarySearch.cs" />
     <Compile Include="$(CommonPath)\CoreLib\System\SpanHelpers.Byte.cs" />
+    <Compile Include="$(CommonPath)\CoreLib\System\SpanHelpers.Char.cs" />
     <Compile Include="$(CommonPath)\CoreLib\System\SpanHelpers.T.cs" />
     <Compile Include="$(CommonPath)\CoreLib\System\Buffers\IRetainable.cs" />
     <Compile Include="$(CommonPath)\CoreLib\System\Buffers\MemoryHandle.cs" />