[build]: check std::system for -[alert|block|wallet]notify

Platforms such as iOs do not support launching a process
through system().

src/util/system.cpp

@@ -1122,6 +1122,7 @@ fs::path GetSpecialFolderPath(int nFolder, bool fCreate)
 }
 #endif
 
+#if defined(HAVE_SYSTEM)
 void runCommand(const std::string& strCommand)
 {
     if (strCommand.empty()) return;
@@ -1133,6 +1134,7 @@ void runCommand(const std::string& strCommand)
     if (nErr)
         LogPrintf("runCommand error: system(%s) returned %d\n", strCommand, nErr);
 }
+#endif
 
 void SetupEnvironment()
 {