Fix Win component build failure

Windows component build with clang has this compile error.

icu_63::uprv_detectWindowsTimeZone' should not add 'dllexport' attribute
[-Werror,-Wdll-attribute-on-redeclaration]
uprv_detectWindowsTimeZone()

Cherry-pick https://github.com/unicode-org/icu/pull/318

TBR=gsathya@chromium.org,ftang@chromium.org

Bug: 913298
Test: Windows component build compiles w/o any error.
Change-Id: I45fc8ac2c1f5fa7821178fea5985c5df6afea850
Reviewed-on: https://chromium-review.googlesource.com/c/1377561


Reviewed-by: Jungshik Shin <jshin@chromium.org>