in libgu mark explicitly all functions that belong to the API

src/runtime/c/gu/utf8.h

@@ -31,13 +31,13 @@ gu_out_utf8(GuUCS ucs, GuOut* out, GuExn* err)
 
 // Helper functions used in other modules
 
-GuUCS
+GU_API_DECL GuUCS
 gu_utf8_decode(const uint8_t** buf);
 
-void
+GU_API_DECL void
 gu_utf8_encode(GuUCS ucs, uint8_t** buf);
 
-void
+GU_API_DECL void
 gu_in_utf8_buf(uint8_t** buf, GuIn* in, GuExn* err);
 
 #endif // GU_UTF8_H_