Include stddef.h in stdatomic.h to define ptrdiff_t

Bug:16874785
Change-Id: I0056a040ead61f32287567449c83025d8fe4df8d
diff --git a/sysroot/usr/include/stdatomic.h b/sysroot/usr/include/stdatomic.h
index febf9ae..7bf04b3 100644
--- a/sysroot/usr/include/stdatomic.h
+++ b/sysroot/usr/include/stdatomic.h
@@ -33,7 +33,8 @@
 #include <sys/cdefs.h>
 #include <sys/types.h>
 #include <stdbool.h>
-#include <stdint.h>  // TODO: Should pollute namespace less
+#include <stddef.h>  // TODO: Should pollute namespace less
+#include <stdint.h>
 
 // NOTE: Defining __CLANG_ATOMICS when __clang__ is defined does not work and results in
 // broken "make checkbuild".