Put the ICU libraries in LIBS_PRIVATE

We don't expose ICU in our public API, so there's no need for end-user
applications to be forced to link to ICU.

Change-Id: Ie7a1cef205b9859be4c6b21f486392d192cf6063
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

src/corelib/tools/tools.pri

@@ -114,8 +114,8 @@ else:include($$PWD/../../3rdparty/zlib_dependency.pri)
 contains(QT_CONFIG,icu) {
     SOURCES += tools/qlocale_icu.cpp
     DEFINES += QT_USE_ICU
-    win32:LIBS += -licuin -licuuc
-    else:LIBS += -licui18n -licuuc
+    win32:LIBS_PRIVATE += -licuin -licuuc
+    else:LIBS_PRIVATE += -licui18n -licuuc
 }
 
 pcre {