package/cairo: fix CVE-2018-19876

Add an upstream patch to fix CVE-2018-19876: cairo 1.16.0, in
cairo_ft_apply_variations() in cairo-ft-font.c, would free memory using a
free function incompatible with WebKit's fastMalloc, leading to an
application crash with a "free(): invalid pointer" error.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: extend commit message]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>