BaseTools: fix an incremental build issue.

BaseTools: fix an incremental build issue.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2137

This patch is going to fix a regression issue of incremental
build which was introduced by the commit 94459080c.
The changing on INF file will not trigger module rebuild.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Steven Shi <steven.shi@intel.com>
Tested-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>