wchar_test.cpp: fix error between comparison signed and unsigned integer

when compile the cts package with aarch64 gcc4.9, will get following error:
bionic/tests/wchar_test.cpp:253:3: required from here
external/gtest/include/gtest/gtest.h:1448:16:
    error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

this change fix it by using static_cast<wchar_t> as suggested by Calin Juravle

Change-Id: I7fb9506e7b84b8a12b9d003458d4f0e78554c3cd
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
1 file changed