configure: Fix SDL2 typo by referencing value

configure: Fix SDL2 typo by referencing value

During configuration of SDL2, variable enable_grub_emu_sdl2 is checked
whether to throw an error message. However, error could not happen
because two unequal strings were compared. Fix this by referencing
value of enable_grub_emu_sdl2, not name.

Fixes: 17d6ac1a7 (emu: Add SDL2 support)
Signed-off-by: Michał Grzelak <mchl.grzlk@gmail.com>
Reviewed-by: Julian Andres Klode <julian.klode@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>