fix mbfl function prototypes

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Broken by 409aa20

As

	(*filter->filter_flush)(filter);
	return filter->flush_function ? (*filter->flush_function)(filter->data) : 0;

flush_function must accept a void * (data)
while filter_flush must accept a mbfl_convert_filter*

ping @alexdowad

the mbfilter.c have to be reviewed, old code have some pointer cast.... changing the type aoid the cast but is obviously wrong.

P.S. used in mailparse extension.

[alexdowad]

This looks better. Thank you for the catch.

[nikic]

/home/travis/build/php/php-src/ext/mbstring/libmbfl/filters/mbfilter_cp5022x.c:590:23: error: assignment from incompatible pointer type [-Werror=incompatible-pointer-types]

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@nikic yes, old code (PHP 7.x) have lot of cast, which seems terribly uggly.
Will re-add them

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travis failure is related to ext/mysqli/tests/mysqli_real_connect_compressed.phpt, not to this PR

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mbstring extension:

Number of tests :  316               314
Tests skipped   :    2 (  0.6%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :    0 (  0.0%) (  0.0%)
Tests passed    :  314 ( 99.4%) (100.0%)

mailparse extension

Number of tests :   15                14
Tests skipped   :    1 (  6.7%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :    0 (  0.0%) (  0.0%)
Tests passed    :   14 ( 93.3%) (100.0%)

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merged as b1c5532