Fix VPATH builds for src/test/ssl targets

Commit b4c4a00ea refactored the gist of the sslfiles target into a
separate makefile in order to override settings in Makefile.global.
The invocation of this this file didn't however include the absolute
path for VPATH builds, resulting in "make clean" failing. Fix by
providing the path to the new makefile.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20211026174152.jjcagswnbhxu7uqz@alap3.anarazel.de
