[OTS] build failure on android_aosp

AOSP's compiler is too strict about signed and unsigned integer
comparison. Add a cast to fix the sign-compare warning.

TEST=compile
R=yusukes@chromium.org

Review URL: https://codereview.chromium.org/77173003

git-svn-id: http://ots.googlecode.com/svn/trunk@107 a4e77c2c-9104-11de-800e-5b313e0d2bf3
diff --git a/src/woff2.cc b/src/woff2.cc
index 1181479..02b181e 100644
--- a/src/woff2.cc
+++ b/src/woff2.cc
@@ -826,7 +826,7 @@
       if (!ReadBase128(file, &src_length)) {
         return OTS_FAILURE();
       }
-    } else if ((flag_byte >> 6) == kShortFlagsContinue) {
+    } else if (static_cast<uint32_t>(flag_byte >> 6) == kShortFlagsContinue) {
       // The compressed data for this table is in a previuos table, so we set
       // the src_length to zero.
       src_length = 0;