sync timestamps also for forwarding headers to generated ones

this wasn't possible when these headers were still generated by syncqt,
as the targets may have been still missing at that time. however, as we
do that now with qmake, forwarding the timestamps is perfectly possible,
and is consistent with what syncqt itself does for "regular" headers.

the immediate problem this solves: when the early creation of the
forwarding headers in qtbase.pro is removed, they get created only when
corelib.pro is processed. their timestamps would be after the timestamps
of the already built bootstrapped libraries. if now the project files of
these libs get re-created, qmake's not conditional-aware dependency scan
would add these headers to the libs' deps, thus causing them to be
re-built. the re-built tools would in turn cause all mocs and thus all
libraries to be re-built. this would be particularly problematic if it
happened between 'make' and 'make install' due to another bug ...

Change-Id: I8d597f1f925369d93aaf3cc6c02e954eeae003a4
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/qt_module_headers.prf

@@ -111,12 +111,14 @@ for (injection, SYNCQT.INJECTIONS) {
     MAIN_FWD = $$INC_PATH/include/$$MODULE_INCNAME/$$fwd_hdr
     MAIN_FWD_CONT = '$${LITERAL_HASH}include "$$relative_path($$dst_hdr, $$dirname(MAIN_FWD))"'
     write_file($$MAIN_FWD, MAIN_FWD_CONT)|error()
+    touch($$MAIN_FWD, $$dst_hdr)
     !git_build: QMAKE_DISTCLEAN += $$MAIN_FWD
     injects = $$member(injects, 2, -1)
     for (inject, injects) {
         CLASS_FWD = $$INC_PATH/include/$$MODULE_INCNAME/$$inject
         CLASS_FWD_CONT = '$${LITERAL_HASH}include "$$fwd_hdr"'
         write_file($$CLASS_FWD, CLASS_FWD_CONT)|error()
+        touch($$CLASS_FWD, $$dst_hdr)
         !git_build: QMAKE_DISTCLEAN += $$CLASS_FWD
     }
 }