MdePkg/PciSegmentInfoLib: Add missing EFIAPI to GetPciSegmentInfo()

All public APIs should have EFIAPI modifier. Somehow PciSegmentInfoLib
missed the EFIAPI modifier.
The patch updates the library header file and NULL instance in MdePkg
to add the missing EFIAPI.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>

MdePkg/Include/Library/PciSegmentInfoLib.h

@@ -3,7 +3,7 @@
 
   This library is consumed by PciSegmentLib to support multiple segment PCI configuration access.
 
-  Copyright (c) 2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2017 - 2020, Intel Corporation. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -28,6 +28,7 @@ typedef struct {
   @retval A callee owned array holding the segment information.
 **/
 PCI_SEGMENT_INFO *
+EFIAPI
 GetPciSegmentInfo (
   UINTN  *Count
   );