Revert "Alpine bugfix for storage size of 'statfsbuf' isn't known"

[jedisct1]

Reverts #168

HAVE_STRUCT_STATVFS and HAVE_STRUCT_STATVFS64 are not defined anywhere.

[lgoio]

This error occurred in an alpine docker on x86_64,
the system is using STATVFS64 according to HAVE_STATVFS64.

After my google research i found this way in libstatgrab/src/libstatgrab/disk_stats.c,
caused by the fallback presumably to STATVFS, im able to compile on alpine.

My configuration:
./configure
--prefix=/usr
--sysconfdir=/etc/pureftpd
--with-tls
--with-puredb
--with-quotas
--with-throttling
--with-rfc2640
--with-paranoidmsg
--with-altlog
--with-cookie
--with-diraliases
--with-bonjour
--with-virtualchroot
--enable-nls
--with-language=english
--without-humor
--with-boring
--without-inetd
--with-uploadscript