Ship the PG18 macOS FDW module as .so (inst handles .dylib)

On macOS PG18, PGXS builds steampipe_postgres_fdw.dylib (DLSUFFIX=.dylib), but
the fdw Makefile `inst` target hardcoded `cp steampipe_postgres_fdw.so`, so the
build-<PLATFORM> copy failed ("No such file or directory") and the draft-release
build aborted (compile+link had succeeded). `inst` now copies the .dylib to
build-<PLATFORM>/steampipe_postgres_fdw.so when present (falling back to .so on
Linux / PG<=17 macOS), matching the .so-pinned control file + the CLI's
FdwBinaryFileName - the proven darwin/arm64 config. Linux unaffected.

Author: kaidaguerre

fdw/Makefile

@@ -58,11 +58,11 @@ inst:
 	mkdir -p ../build-${PLATFORM}
 	rm -f ../build-${PLATFORM}/*
 
-	cp steampipe_postgres_fdw.so ../build-${PLATFORM}
+	@if [ -f steampipe_postgres_fdw.dylib ]; then cp steampipe_postgres_fdw.dylib ../build-${PLATFORM}/steampipe_postgres_fdw.so; else cp steampipe_postgres_fdw.so ../build-${PLATFORM}/steampipe_postgres_fdw.so; fi
 	cp steampipe_postgres_fdw.control ../build-${PLATFORM}
 	cp steampipe_postgres_fdw--1.0.sql ../build-${PLATFORM}
-	
-	rm steampipe_postgres_fdw.so
+
+	rm -f steampipe_postgres_fdw.so steampipe_postgres_fdw.dylib
 	rm steampipe_postgres_fdw.a
 	rm steampipe_postgres_fdw.h