BaseTools: Fixed the bug of multi-thread genffs for override inf

BaseTools: Fixed the bug of multi-thread genffs for override inf

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1302
If there is a inf override, and multi-thread genffs is enabled, build
will fail.

This patch is going to fix this bug.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Steven Shi <steven.shi@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>