UefiCpuPkg/SmmCpuFeaturesLib: Move multi-instance function decl to header

FinishSmmCpuFeaturesInitializeProcessor() is a multi-instance
internal library function that is currently not declared in a
header file but embedded in "SmmCpuFeaturesLib.c".

This change cleans up the declaration moving it to a new header
file "CpuFeaturesLib.h" and removing the local declaration in
"SmmCpuFeaturesLib.c".

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210217213227.1277-2-mikuback@linux.microsoft.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
[lersek@redhat.com: replace the guard macro "_CPU_FEATURES_LIB_H_" with
 "CPU_FEATURES_LIB_H_", for fixing ECC 8003, per commit 6ffbb3581a]

UefiCpuPkg/Library/SmmCpuFeaturesLib/CpuFeaturesLib.h

@@ -0,0 +1,22 @@
+/** @file
+  Internal library function definitions.
+
+  Copyright (c) Microsoft Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#ifndef CPU_FEATURES_LIB_H_
+#define CPU_FEATURES_LIB_H_
+
+/**
+  Internal worker function that is called to complete CPU initialization at the
+  end of SmmCpuFeaturesInitializeProcessor().
+
+**/
+VOID
+FinishSmmCpuFeaturesInitializeProcessor (
+  VOID
+  );
+
+#endif