CMake Build: Use shared precompile headers

This will make the build directory size smaller, as only two
pch files will be generated. Not one for every target.

Change-Id: I3eec91e7536eab1c62bff8843f075f0ef7b5fff6
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/tracing/CMakeLists.txt

@@ -6,6 +6,7 @@ endif()
 
 add_qtc_library(Tracing
   DEPENDS Utils Qt5::Qml Qt5::Quick
+  PUBLIC_DEPENDS Qt5::Widgets
   SOURCES ${TEST_SOURCES}
     flamegraph.cpp flamegraph.h
     flamegraphattached.h