Fix the install rule for snowball_create.sql.

Fix the install rule for snowball_create.sql.

This file could be in the current (build) directory if we just
built it.  However, when installing from a VPATH build from a
tarball, it will exist in the source directory and gmake will
therefore not rebuild it.  Use the $< macro to find out where
gmake found it.

Oversight in b3a0d8324, which also exposes a buildfarm testing gap:
we test install from VPATH builds from bare source trees, but not
from tarballs.

Per report from Christoph Berg.

Discussion: https://postgr.es/m/ZGzEAqjxkkoY3ooH@msg.df7cb.de