package/gzip: fix musl build

Inspired by http://patchwork.openembedded.org/patch/111691/
Adding "gl_cv_func_fflush_stdin=yes" was sufficient to fix the build.

Fixes
http://autobuild.buildroot.net/results/56d/56d926fcf8ca53cddb9106617e3b852d9c9a4eeb/
http://autobuild.buildroot.net/results/4b5/4b5a6196b686030cf7fa9706508fd99f81a81049/
http://autobuild.buildroot.net/results/568/568c1c05800d6bdd6457845e998a598e354afdfd/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>