CMake: Fix potential build failure on 64-bit Windows

CMakeLists.txt

@@ -258,6 +258,9 @@ if(BUILD_PYTHON)
     if(APPLE)
         set(PYBIND_LIBS "-undefined dynamic_lookup")
     elseif(WIN32)
+        if(CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")
+            add_definitions(-DMS_WIN64)
+        endif()
         set(PYBIND_LIBS "${PYTHON_LIBRARY}")
     endif()
     target_link_libraries(retro retro-base ${PYBIND_LIBS} ${STATIC_LDFLAGS})