[lldb][windows] fix undeclared identifier error GetModulePath (#180989)

`LLDB_PYTHON_DLL_RELATIVE_PATH` can be unset while
`LLDB_PYTHON_RUNTIME_LIBRARY_FILENAME` is, since it's set automatically
by CMake. This situation causes a build failure which this patch fixes.

This issue was introduced in
https://github.com/llvm/llvm-project/pull/180784.

GitOrigin-RevId: 2fd5479f2c78780c76424d4d918d69b32bf1573c