src/backend/Makefile: Add -I. to make enable vpath builds

Debian is building the backend module for several PostgreSQL versions in
parallel, using different build directories. This fix enables this "out
of tree" vpath build.

Branch
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REL_2_2_STABLE

Details
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https://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=c49ce8ae7bd2361e03843c0302b3f916db32aef0
Author: Christoph Berg <myon at debian.org>

Modified Files
--------------
src/backend/Makefile | 1 +
1 file changed, 1 insertion(+)