windows signing: also wait for/depend on extras (gallery)

windows signing: also wait for/depend on extras (gallery)

Otherwise, doing 'make' with --enable-windows-build-signing

fails with

ERROR: SignTool Error: The file is being used by another process.
SignTool Error: An error occurred while attempting to sign: C:/cygwin64/home/buildslave/r/instdir/program/Argon2OptDll.dll

make[1]: *** [C:/cygwin64/home/buildslave/source/libo-core/postprocess/CustomTarget_signing.mk:31: C:/cygwin64/home/buildslave/r/workdir/CustomTarget/postprocess/signing/signing.done] Error 1
make: *** [Makefile:289: build] Error 2

Change-Id: I9d0793f3e42577a47b92025d5670caa44fbd912e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/161388
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>