gsl.vapi: Fix attribute typos
gsl.vapi: Fix attribute typos

At the moment, the are a few CCode attributes containing typos in the
bindings for the GNU Scientific Library. This patch rectifies the ones
that valac explicitly complains about.

https://bugzilla.gnome.org/show_bug.cgi?id=772004

GNOME/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala