util/pgtblgen: Improve compatibility

Fix build on Debian/jessie

Change-Id: I987e7a03441b40ab06ccd54a21e38aac81a1c28d
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48004
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

util/pgtblgen/pgtblgen.c

@@ -9,6 +9,7 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <inttypes.h>
+#include <getopt.h>
 
 static void usage(char *argv[])
 {