CryptoPkg/BaseHashApiLib: Change PcdHashApiLibPolicy type to FixedAtBuild

CryptoPkg/BaseHashApiLib: Change PcdHashApiLibPolicy type to FixedAtBuild

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2511

This commit changes the PCD PcdHashApiLibPolicy to the type
PcdsFixedAtBuild so as to be able to optimize away the unused hashing
algorithms in HashApiLib instance used by a driver.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Amol N Sukerkar <amol.n.sukerkar@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>