pcp: make the ToString method const

src/common/pcp.cpp

@@ -512,7 +512,7 @@ std::variant<MappingResult, MappingError> PCPRequestPortMap(const PCPMappingNonc
     return MappingResult(PCP_VERSION, CService(internal, port), CService(external_addr, external_port), lifetime_ret);
 }
 
-std::string MappingResult::ToString()
+std::string MappingResult::ToString() const
 {
     Assume(version == NATPMP_VERSION || version == PCP_VERSION);
     return strprintf("%s:%s -> %s (for %ds)",

src/common/pcp.h

@@ -40,7 +40,7 @@ struct MappingResult {
     uint32_t lifetime;
 
     //! Format mapping as string for logging.
-    std::string ToString();
+    std::string ToString() const;
 };
 
 //! Try to open a port using RFC 6886 NAT-PMP. IPv4 only.