Fix embedded Linux debug build

The icu header files are picked up from
src/3rdparty/chromium/third_party/icu, but the syroot's icu libs
were linked.
Do not claim to support linking against the sysroot's icu.
This never worked properly in 5.5.

Task-number: QTBUG-49693
Change-Id: I7dca37ecefbc5b1dabfadf86540442b8face8347
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>