Added comment about how to disassemble JIT output.

upb/pb/decoder_x86.dasc

@@ -682,6 +682,8 @@ void upb_decoder_makejit(upb_decoder *d) {
 
   mprotect(d->jit_code, d->jit_size, PROT_EXEC | PROT_READ);
 
+  // View with: objdump -M intel -D -b binary -mi386 -Mx86-64 /tmp/machine-code
+  // Or: ndisasm -b 64 /tmp/machine-code
   FILE *f = fopen("/tmp/machine-code", "wb");
   fwrite(d->jit_code, d->jit_size, 1, f);
   fclose(f);

upb/upb.h

@@ -14,6 +14,7 @@
 #include <stdint.h>
 #include <stdio.h>  // only for size_t.
 #include <assert.h>
+#include <stdarg.h>
 #include "descriptor_const.h"
 #include "atomic.h"