Fix build with libunistring >= 1.4.
https://github.com/dankamongmen/notcurses/pull/2897
--- src/lib/internal.h.orig
+++ src/lib/internal.h
@@ -1,10 +1,6 @@
 #ifndef NOTCURSES_INTERNAL
 #define NOTCURSES_INTERNAL
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include "version.h"
 #include "builddef.h"
 #include "compat/compat.h"
@@ -414,6 +410,10 @@ struct blitset {
 
 #include "blitset.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void reset_stats(ncstats* stats);
 void summarize_stats(notcurses* nc);