theme-proof tab size warning

[IgnorantGuru]

bbbf9bea is a nice addition - looks good. But adds a make warning:

main-window.c: In function ‘fm_main_window_create_tab_label’:
main-window.c:2540:13: warning: passing argument 2 of ‘gtk_style_context_add_provider’ from incompatible pointer type [enabled by default]
In file included from /usr/include/gtk-3.0/gtk/gtkgradient.h:26:0,
                 from /usr/include/gtk-3.0/gtk/gtk.h:110,
                 from pcmanfm.h:25,
                 from main-window.c:13:
/usr/include/gtk-3.0/gtk/gtkstylecontext.h:717:6: note: expected ‘struct GtkStyleProvider *’ but argument is of type ‘struct GtkCssProvider *’

[BwackNinja]

Warning is gone now, closing.