rootdev: include missing header file.

This adds <sys/sysmacros.h> because the
inclusion of <sys/sysmacros.h> by <sys/types.h> is deprecated since
glibc 2.25.

BUG=chromium:865304
TEST=emerge-samus rootdev with glibc 2.27

Change-Id: Ic54e089659d620a862720b9882235117eb8487b5
Reviewed-on: https://chromium-review.googlesource.com/1143004
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

main.c

diff --git a/main.c b/main.c
index ca99723..8473fa4 100644
--- a/main.c
+++ b/main.c
@@ -13,6 +13,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <unistd.h>
 

rootdev.c

diff --git a/rootdev.c b/rootdev.c
index ce1a970..037140c 100644
--- a/rootdev.c
+++ b/rootdev.c
@@ -19,6 +19,7 @@
 #include <string.h>
 #include <sys/ioctl.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <sys/vfs.h>
 #include <unistd.h>