BaseTools: Fix the bug to re-build uni file for Library

The root cause is Module's self.CanSkip() is before LibraryAutoGen,
then when a uni file of library is changed, Module's self.CanSkip() is
still true which cause the library is not regenerated.
This patch change Module's self.CanSkip() after LibraryAutoGen.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=759
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>