ICU-21249 Fix common/uniquecharstr.h to pass the internal header check: include uassert.h needed.
diff --git a/icu4c/source/common/uniquecharstr.h b/icu4c/source/common/uniquecharstr.h
index 7e871dc..31110a3 100644
--- a/icu4c/source/common/uniquecharstr.h
+++ b/icu4c/source/common/uniquecharstr.h
@@ -8,6 +8,7 @@
 #define __UNIQUECHARSTR_H__
 
 #include "charstr.h"
+#include "uassert.h"
 #include "uhash.h"
 
 U_NAMESPACE_BEGIN