minor fix

Author: karasikov

include/sdsl/ram_filebuf.hpp

@@ -15,7 +15,7 @@ class ram_filebuf : public std::streambuf
         void pbump64(std::ptrdiff_t);
 
     public:
-        ~ram_filebuf() override {}
+        ~ram_filebuf() {}
 
         ram_filebuf() {}
         ram_filebuf(std::vector<char>& ram_file);