You need to link with d3dcompiler.lib to use the D3DCompile().
The latest code on master in imgui_impl_dx10.cpp automatically pulls this library in if you use Visual Studio
#ifdef _MSC_VER
#pragma comment(lib, "d3dcompiler") // Automatically link with d3dcompiler.lib as we are using D3DCompile() below.
#endif
Else add d3dcompiler.lib to your linker.
Note that you would find this answer if you googled “Error LNK2019 unresolved external symbol D3DCompile referenced”.