kernel_efivars.c: fix mismatch between UNIX and EFI time

The EFI variable code is failing in January of every year.  This is
because of a mismatch between EFI_TIME and struct tm.  The month in
EFI_TIME is 1-12 and in struct tm it's 0-11 meaning that January is an
invalid month for EFI_TIME.

Fix this by adding one

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>