Build fail with -Werror=format-security

[Nobuhiro Iwamatsu]

Build fail with -Werror=format-security.

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export.c: In function ‘write_line’:
export.c:94:3: warning: format ‘%i’ expects argument of type ‘int’, but argument 6 has type ‘size_t’ [-Wformat]
export.c: In function ‘export’:
export.c:125:25: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
export.c: In function ‘main’:
export.c:206:10: error: format not a string literal and no format arguments [-Werror=format-security]
export.c: In function ‘export’:
export.c:168:7: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]
cc1: some warnings being treated as errors
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