lib: Neuter _VTE_GNUC_NONNULL when compiling libvte
lib: Neuter _VTE_GNUC_NONNULL when compiling libvte

thestinger/vte-ng,saitoha/vte-sixel,saitoha/vte-sixel,thestinger/vte-ng,saitoha/vte-sixel,thestinger/vte-ng