Bump xtb submodule to 6.4.0 version (#43)

awvwgk · web-flow · commit bc55eeb97194 · 2021-02-01T09:36:57.000+01:00
diff --git a/subprojects/xtb b/subprojects/xtb
@@ -1 +1 @@
-Subproject commit 5b13467bd63a2a4dbd5b339ed44d0fa026b6f0af
+Subproject commit d4b70c2b361067f291e81c3df1d1c31c28d01cb4
