gnucash maint: Fix build with cmake older than 3.18

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash/commit/d95a15fb (commit)
	from  https://github.com/Gnucash/gnucash/commit/227159d8 (commit)



commit d95a15fb4342653b4afcb7fa8717cfadde99fbee
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Fri Dec 17 09:36:26 2021 +0100

    Fix build with cmake older than 3.18
    
    3.18 introduced 'cmake -E cat', but we still have to support
    versions starting from 3.14.



Summary of changes:
 gnucash/gschemas/CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)