vs2015: Add missing file to test_opus_encode.

Signed-off-by: Ralph Giles <giles@thaumas.net>
diff --git a/win32/VS2015/test_opus_encode.vcxproj b/win32/VS2015/test_opus_encode.vcxproj
index e5789cc..41019c7 100644
--- a/win32/VS2015/test_opus_encode.vcxproj
+++ b/win32/VS2015/test_opus_encode.vcxproj
@@ -51,6 +51,7 @@
     </ProjectConfiguration>
   </ItemGroup>
   <ItemGroup>
+    <ClCompile Include="..\..\tests\opus_encode_regressions.c" />
     <ClCompile Include="..\..\tests\test_opus_encode.c" />
   </ItemGroup>
   <ItemGroup>