codegen: Mark needle parameter of internal array-contains methods as const
codegen: Mark needle parameter of internal array-contains methods as const

This will avoid some -Wdiscarded-qualifiers warnings.

Fixes https://gitlab.gnome.org/GNOME/vala/issues/504

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