DuetPkg DxeIpl and EfiLdr: Add the missing EFIAPI for the function

The function with the variable parameters should have EFIAPI.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

DuetPkg/DxeIpl/Debug.c

@@ -52,6 +52,7 @@ ClearScreen (
 }
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...

DuetPkg/DxeIpl/Debug.h

@@ -27,6 +27,7 @@ PrintHeader (
   );
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...

DuetPkg/EfiLdr/Debug.c

@@ -49,6 +49,7 @@ ClearScreen (
 }
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...

DuetPkg/EfiLdr/Debug.h

@@ -27,6 +27,7 @@ PrintHeader (
   );
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...