src: add missing #include <unicode/ustring.h>

* We use these functions that are declared in <unicode/ustring.h>

  u_strFromUTF8()
    u_strToUTF8()

* At present, <unicode/ustring.h> is indirectly included, but this will
likely change in future ICUs. Adding this header has been the right
thing to do for many years, so it is backwards compatible.

Fixes: nodejs#11753
PR-URL: nodejs#11754
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

src/node_i18n.cc

@@ -43,6 +43,7 @@
 #include <unicode/ulocdata.h>
 #include <unicode/uvernum.h>
 #include <unicode/uversion.h>
+#include <unicode/ustring.h>
 
 #ifdef NODE_HAVE_SMALL_ICU
 /* if this is defined, we have a 'secondary' entry point.