gh-142779: Initialize reserved field for proper padding (#142780)

The jitdump specification specifies a reserved field for padding.

Initialize it so no garbage data is embedded in the jitdump files.

Python/perf_jit_trampoline.c

@@ -398,6 +398,7 @@ static void perf_map_jit_write_header(int pid, FILE* out_file) {
     header.version = 1;                           // Current jitdump version
     header.size = sizeof(Header);                 // Header size for validation
     header.elf_mach_target = GetElfMachineArchitecture();  // Target architecture
+    header.reserved = 0;                          // padding reserved for future use
     header.process_id = pid;                      // Process identifier
     header.time_stamp = get_current_time_microseconds();   // Creation time
     header.flags = 0;                             // No special flags currently used