pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5df5bea29070b420452bdb257c3dec1cf0419fca

Modified Files
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src/backend/snowball/Makefile | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)