<tools/gyp> FIXUP: Allow letting qmake do the link step

ld expects the -l switches to appear after the dependent object on
the command line. It works if we use QT_PRIVATE to trigger Qt being
added to the link line since it would be appended to LIBS_PRIVATE as
well. But QT will append to LIBS, which is listed first in the
Makefile and would cause undefined symbol errors by following .o
files.

Fix the issue by adding .o files to OBJECTS instead of LIBS_PRIVATE.
This will only work if all Qt-dependent code is listed in the
top-level gyp file, but we've been keeping it layered that way since
the beginning and it should be acceptable to require it.

Change-Id: I3b655ce8d6525fb986ca87fb9c2863a239ebf991
Reviewed-by: Andras Becsi <andras.becsi@digia.com>