Allow changing network upgrade parameters on regtest

Derived from upstream commit 56c87e92110f05d7452f1e85bf755246ffc77206:
    Allow changing BIP9 parameters on regtest

src/utilstrencodings.h

@@ -24,6 +24,7 @@
 
 std::string SanitizeFilename(const std::string& str);
 std::string SanitizeString(const std::string& str);
+std::string HexInt(uint32_t val);
 std::vector<unsigned char> ParseHex(const char* psz);
 std::vector<unsigned char> ParseHex(const std::string& str);
 signed char HexDigit(char c);