Qt5CoreMacros: append the --compiler-flavor=msvc flag

Previously, the flags were replaced rather than appended to, losing -I
flags and causing `#include` directives to fail.

Change-Id: I74609e891ea327a8136c8075ab13176fc85ab111
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/Qt5CoreMacros.cmake

@@ -91,7 +91,7 @@ macro(QT5_GET_MOC_FLAGS _moc_flags)
         set(${_moc_flags} ${${_moc_flags}} -DWIN32)
     endif()
     if (MSVC)
-        set(${_moc_flags} --compiler-flavor=msvc)
+        set(${_moc_flags} ${${_moc_flags}} --compiler-flavor=msvc)
     endif()
 endmacro()