build: reintroduce build/.empty for convenience
build: reintroduce build/.empty for convenience

This fixes the bug introduced by 3ea3f247, which deleted .empty file
unintentionally.

Without this, we need to call mkdir to create the "build" directory,
and it's just incovenient, especially on CI.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit