Make <sys/user.h> compile on AArch64.

Previously, AArch64 used uint64_t without including <stdint.h>.

Bug: http://b/28178111
Change-Id: I8a497765d835f32b456f7eb925a6760e523163c4
diff --git a/libc/include/sys/user.h b/libc/include/sys/user.h
index 3312981..13fecf9 100644
--- a/libc/include/sys/user.h
+++ b/libc/include/sys/user.h
@@ -31,6 +31,7 @@
 
 #include <sys/cdefs.h>
 #include <stddef.h> /* For size_t. */
+#include <stdint.h>
 
 __BEGIN_DECLS