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>

README.chromium

@@ -282,5 +282,4 @@ D. Local Modifications
     https://unicode-org.atlassian.net/browse/ICU-20302
   - Fix:
     https://github.com/unicode-org/icu/pull/315
-
-   
+    https://github.com/unicode-org/icu/pull/318

patches/win7_tz.patch

@@ -29,3 +29,16 @@ index 5e9ac0d2..8a143d9e 100644
  
      if (U_FAILURE(status)) {
          return nullptr;
+diff --git a/source/common/wintz.h b/source/common/wintz.h
+index f98b1779..cd8565ee 100644
+--- a/source/common/wintz.h
++++ b/source/common/wintz.h
+@@ -28,7 +28,7 @@ U_CDECL_BEGIN
+ typedef struct _TIME_ZONE_INFORMATION TIME_ZONE_INFORMATION;
+ U_CDECL_END
+ 
+-U_CFUNC const char* U_EXPORT2
++U_INTERNAL const char* U_EXPORT2
+ uprv_detectWindowsTimeZone();
+ 
+ #endif /* U_PLATFORM_USES_ONLY_WIN32_API  */

source/common/wintz.h

@@ -28,7 +28,7 @@ U_CDECL_BEGIN
 typedef struct _TIME_ZONE_INFORMATION TIME_ZONE_INFORMATION;
 U_CDECL_END
 
-U_CFUNC const char* U_EXPORT2
+U_INTERNAL const char* U_EXPORT2
 uprv_detectWindowsTimeZone();
 
 #endif /* U_PLATFORM_USES_ONLY_WIN32_API  */