externals: submodule tzdb_to_nx

Fix for Flatpak being unable to download during CMake configure.

.gitmodules

@@ -52,3 +52,6 @@
 [submodule "libadrenotools"]
 	path = externals/libadrenotools
 	url = https://github.com/bylaws/libadrenotools
+[submodule "tzdb_to_nx"]
+	path = externals/nx_tzdb/tzdb_to_nx
+	url = https://github.com/lat9nq/tzdb_to_nx.git

externals/nx_tzdb/tzdb_to_nx

@@ -0,0 +1 @@
+Subproject commit 34df65eff295c2bd9ee9e6a077d662486d5cabb3