cursor: add one more directory to XCURSORPATH

The user may install cursor themes manually, and the desktop environment
may provide a special directory for storing them.  For instance, GTK puts
those themes into ~/.local/share/icons, and many Linux distributions suggest
using that directory for user-specific themes.  However, users of
libwayland-cursor cannot load these themes using the API provided by the
library because the latter does not look into that directory.

This patch adds ~/.local/share/icons to the search path, so user-specific
themes can be loaded through the API provided by libwayland-cursor.

Signed-off-by: Alexander Dunaev <adunaev@igalia.com>
Reviewed-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Simon Ser <contact@emersion.fr>