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/Library/BasePciSegmentInfoLibNull/PciSegmentInfoLib.c

@@ -2,7 +2,7 @@
   Default PCI Segment Information Library that returns one segment whose
   segment base address equals to PcdPciExpressBaseAddress.
 
-  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
 
 **/
@@ -21,6 +21,7 @@
   @retval A callee owned array holding the segment information.
 **/
 PCI_SEGMENT_INFO *
+EFIAPI
 GetPciSegmentInfo (
   UINTN  *Count
   )