Fix windows build of AAPT2

Change-Id: Ib8e1a4322510b582e9600a08d3118842c9abc73c
diff --git a/tools/aapt2/Util.h b/tools/aapt2/Util.h
index 2de9568..510ed76 100644
--- a/tools/aapt2/Util.h
+++ b/tools/aapt2/Util.h
@@ -88,7 +88,7 @@
 
 inline ::std::function<::std::ostream&(::std::ostream&)> formatSize(size_t size) {
     return [size](::std::ostream& out) -> ::std::ostream& {
-        constexpr size_t K = 1024;
+        constexpr size_t K = 1024u;
         constexpr size_t M = K * K;
         constexpr size_t G = M * K;
         if (size < K) {