Makefile: remove accidental recipe prefix in conditional

Back in 728b9ac0c3 (Makefile(s): avoid recipe prefix in conditional statements, 2024-04-08), we prepared our Makefiles for a forthcoming change in upstream Make that would ban the recipe prefix within a conditional statement by replacing tabs (the prefix) with eight spaces. In b9d6f64393 (compat/zlib: allow use of zlib-ng as backend, 2025-01-28), a handful of recipe prefix characters were introduced in a conditional statement ('ifdef ZLIB_NG'), causing 'make' to fail on my system, which uses GNU Make 4.4.90. Remove the recipe prefix characters by replacing them with the same script as is mentioned in 728b9ac0c3. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>