[C++] Fix err_semicolon_in_output.mk
diff --git a/strutil.h b/strutil.h
index b0ea67b..848fc42 100644
--- a/strutil.h
+++ b/strutil.h
@@ -121,6 +121,7 @@
 
 size_t FindOutsideParen(StringPiece s, char c);
 size_t FindTwoOutsideParen(StringPiece s, char c1, char c2);
+size_t FindThreeOutsideParen(StringPiece s, char c1, char c2, char c3);
 
 size_t FindEndOfLine(StringPiece s, size_t e, size_t* lf_cnt);