gvplugin_load: use a 'strview_t' for 'dep'

gvplugin_load: use a 'strview_t' for 'dep'

Similar to the previous commit, this string is either `NULL` or terminated by a
`'\0'`, so there is little advantage in using a `strview_t` over a
`const char*`. But this will make it more consistent with surrounding code after
upcoming changes.