select_x13_binary.m: use direct path under Windows

Related to Dynare/dynare#1878 (comment 18606)

src/utilities/x13/select_x13_binary.m

@@ -23,8 +23,13 @@ if nargin < 1
     warn_only = false;
 end
 
-dseries_src_root = strrep(which('initialize_dseries_class'),'initialize_dseries_class.m','');
-dseries_x13_root = sprintf('%s%s%s%s%s%s%s', dseries_src_root, '..', filesep(), 'externals', filesep(), 'x13', filesep());
+if ispc()
+    dseries_src_root = strrep(which('initialize_dseries_class'),'src\initialize_dseries_class.m','');
+    dseries_x13_root = sprintf('%s%s%s%s%s', dseries_src_root,'externals', filesep(), 'x13', filesep());
+else
+    dseries_src_root = strrep(which('initialize_dseries_class'),'initialize_dseries_class.m','');
+    dseries_x13_root = sprintf('%s%s%s%s%s%s%s', dseries_src_root, '..', filesep(), 'externals', filesep(), 'x13', filesep());
+end
 
 if ismac()
     x13_binary = sprintf('%s%s%s', dseries_x13_root, 'macOS', filesep());