staging/easycap: fix missing backslash in ifdef statement

the backslash was removed by mistake in the patch
'staging:easycap: drop redundant backslashes from the code'
this breaks compilation only when EASYCAP_NEEDS_ALSA is not set

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>