Fixed bug where highlighting on inactive windows wouldn't refresh.

This seems to be an omission/regression from #afb75adc, where inactive
highlight updating was accidentally removed when fixing another bug.

Solution: Add back the deleted statement. closes #1339

Repro steps:
1. Install some theme that depends on the background color
   (Soares/base16.nvim has a bunch)
2. `set background=dark` in your vimrc, and `colorscheme` one of the
   aforementioned schemes.
3. Open a split window. Note the colors on the inactive window's airline.
4. `set background=light` manually. note the colors on the inactive
   window's airline. Note how they have not updated. (In particular,
   airline_c_inactive has updated, but all the other inactive groups
   have not.)
5. Enter the inactive window. Exit the inactive window. Observe that the
   colors are now correct (showing that it is in fact a problem with the
   airline load_theme code, and not with the theme).

It seems strange that the code as written only expects
airline_c_inactive to have styling; perhaps there is some norm that
themes are supposed to handle inactive windows in a particular way? For
the record, my theme dis omething like this:

```
let s:IA1   = s:airlist('similar1', 'similar2')
let s:IA2   = s:airlist('similar1', 'similar2')
let s:IA3   = s:airlist('similar1', 'similar2')
let g:airline#themes#{s:palette}#palette.inactive = airline#themes#generate_color_map(s:IA1, s:IA2, s:IA3)
let g:airline#themes#{s:palette}#palette.inactive.airline_warning = s:airlist('base', 'contrast3')
let g:airline#themes#{s:palette}#palette.inactive.airline_error = s:airlist('base', 'antibase')
```

autoload/airline/highlighter.vim

@@ -108,6 +108,7 @@ function! airline#highlighter#load_theme()
   for winnr in filter(range(1, winnr('$')), 'v:val != winnr()')
     call airline#highlighter#highlight_modified_inactive(winbufnr(winnr))
   endfor
+  call airline#highlighter#highlight(['inactive'])
   if getbufvar( bufnr('%'), '&modified'  )
     call airline#highlighter#highlight(['normal', 'modified'])
   else