x11/libinput: Fix build on GCC-based architectures

Fix build of x11/libinput on GCC-based architectures. gnu90 is an alias for
gnu89, but is not understood by all versions of GCC, most notably, our base
version. Switch to use gnu89 instead.
While here, adjust patch context a bit.

A similar fix has been merged upstream.

PR: 235361
Submitted by: Jan Beich (fix)
Reported by: Piotr Kubaj (PR)
MFH: 2019Q1
X-MFH-note: If applicable, 2019Q1 might have old libinput version