BaseTools: Fixed the build fail on Linux with --genfds-multi-thread

BaseTools: Fixed the build fail on Linux with --genfds-multi-thread

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1302

If GenSec input file not exist, the related command will fail and make
will stop on Linux. GenSec input file is allow to be non-existent.

This patch is to let "make" continue if gensec input file not exist.

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