[JFFS2] Add declaration of jffs2_lzo_{init,exit} to compr.h

fs/jffs2/compr.c: In function ‘jffs2_compressors_init’:
fs/jffs2/compr.c:320: warning: implicit declaration of function ‘jffs2_lzo_init’
fs/jffs2/compr.c: In function ‘jffs2_compressors_exit’:
fs/jffs2/compr.c:346: warning: implicit declaration of function ‘jffs2_lzo_exit’

Signed-off-by: David Woodhouse <dwmw2@infradead.org>