Fix includes for GCC 8.2

Here's what the compiler from Linaro ((GNU Toolchain for the A-profile
Architecture 8.2-2018.11 (arm-rel-8.26)) 8.2.1 20180802) says:

  In file included from src/datastore_access.cpp:9:
  src/datastore_access.hpp:22:18: error: 'map' in namespace 'std' does not name a template type
       virtual std::map<std::string, leaf_data_> getItems(const std::string& path) = 0;
                    ^~~
  src/datastore_access.hpp:22:13: note: 'std::map' is defined in header '<map>'; did you forget to '#include <map>'?
  src/datastore_access.hpp:13:1:
  +#include <map>

  src/datastore_access.hpp:22:13:
       virtual std::map<std::string, leaf_data_> getItems(const std::string& path) = 0;
               ^~~

Change-Id: If469110df031b5bf2e94c82d520838886541f31d
diff --git a/src/datastore_access.hpp b/src/datastore_access.hpp
index 9cd6a70..7972523 100644
--- a/src/datastore_access.hpp
+++ b/src/datastore_access.hpp
@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <map>
 #include <string>
 #include "ast_values.hpp"