Windows: Add missing header files to MSVC projects

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
diff --git a/msvc/hotplugtest_2013.vcxproj b/msvc/hotplugtest_2013.vcxproj
index 4e71963..2fad4a7 100644
--- a/msvc/hotplugtest_2013.vcxproj
+++ b/msvc/hotplugtest_2013.vcxproj
@@ -72,6 +72,9 @@
     <ClCompile Include="..\examples\hotplugtest.c" />
   </ItemGroup>
   <ItemGroup>
+    <ClInclude Include="..\libusb\libusb.h" />
+  </ItemGroup>
+  <ItemGroup>
     <ProjectReference Include=".\libusb_static_2013.vcxproj">
       <Project>{349ee8f9-7d25-4909-aaf5-ff3fade72187}</Project>
       <ReferenceOutputAssembly>false</ReferenceOutputAssembly>