Always build neighbor domain decomposition source

Author: Fei Yang

source/source_cell/module_neighlist/CMakeLists.txt

@@ -1,14 +1,11 @@
 set(neighbor_search_sources
     bin_manager.cpp
+    domain_decomposition.cpp
     neighbor_search.cpp
     page_allocator.cpp
     unitcell_lite.cpp
 )
 
-if(ENABLE_MPI)
-    list(APPEND neighbor_search_sources domain_decomposition.cpp)
-endif()
-
 add_library(
     neighbor_search
     OBJECT