strptime: fix "differ in signedness" warnings on GCC
strptime: fix "differ in signedness" warnings on GCC

This patch addresses the following warnings:

    warning: pointer targets in passing argument 1 of ‘_flb_strptime’
    differ in signedness

This kind of warnings occur, due to OpenBSD's strptime converting
the input string into unsigned chars (at line 141).

Evidently this conversion has been introduced to avoid string
comparison bugs, but it causes trivial interface mismatches.

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

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