staging/lustre/osc: extern declare osc_caches in a header

This avoids frowned upon extern in the C file, and also
shuts down a sparse warning of
drivers/staging/lustre/lustre/osc/osc_dev.c:55:22: warning: symbol 'osc_caches' was not declared. Should it be static?

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>