build: fix Windows compilation.

src/lol-core.vcxproj

@@ -76,6 +76,7 @@
     </ClCompile>
   </ItemDefinitionGroup>
   <ItemGroup>
+    <ClCompile Include="3rdparty\imgui\imgui.cpp" />
     <ClCompile Include="3rdparty\imgui\imgui_demo.cpp" />
     <ClCompile Include="3rdparty\imgui\imgui_draw.cpp" />
     <ClCompile Include="3rdparty\imgui\imgui_widgets.cpp" />