util/nvmutil: use even older define for pread

with this new define, we can target even older systems
from the late 90s.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -15,7 +15,9 @@
  * -Os -Wall -Wextra -Werror -pedantic -std=c99
  */
 
-#define _POSIX_C_SOURCE 200809L
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 500
+#endif
 
 #ifdef __OpenBSD__
 #include <sys/param.h>