Add jsonapi.c to Mkvcbuild.pm's @pgcommonallfiles.
My recent commit
beb4699091e9fab1c5f465056bef35c9ddf7f9fc caused
some buildfarm breakage, as reported by Tom Lane. Try to repair.
This fix is extracted from a larger patch by Andrew Dunstan.
Discussion: http://postgr.es/m/
8440ddc9-8347-ca64-1405-
845d10e054cd@2ndQuadrant.com
Discussion: http://postgr.es/m/14178.
1580312751@sss.pgh.pa.us