Add #include guards to base64.h

src/base64.h

+#ifndef BASE64_HEADER
+#define BASE64_HEADER
+
 #include <string>
 
 bool base64_is_valid(std::string const& s);
 std::string base64_encode(unsigned char const* , unsigned int len);
 std::string base64_decode(std::string const& s);
+
+#endif // BASE64_HEADER
\ No newline at end of file