efivars: add missing parameter to efi_pstore_read()

In the case where CONFIG_PSTORE=n, the function efi_pstore_read() doesn't
have the correct list of parameters. This patch provides a definition
of efi_pstore_read() with 'char **buf' added to fix this warning:
"drivers/firmware/efivars.c:609: warning: initialization from".

problem introduced in commit f6f82851

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>