Stop #include-ing wcstringutil.h in flog.h

This is a header dependency that we can break.

src/env.cpp

@@ -33,6 +33,7 @@
 #include "proc.h"
 #include "reader.h"
 #include "termsize.h"
+#include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 /// Some configuration path environment variables.