MdePkg/Pldm.h: Add define for the PLDM response flag

The PLDM protocol uses Request bit to help differentiate between PLDM
request and response messages.
Currently the Pldm.h header only have a flag for the request message.
Add a flag for the response message as well.

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>

MdePkg/Include/IndustryStandard/Pldm.h

@@ -40,6 +40,7 @@ typedef struct  {
 typedef PLDM_MESSAGE_HEADER PLDM_REQUEST_HEADER;
 
 #define PLDM_MESSAGE_HEADER_IS_REQUEST        1
+#define PLDM_MESSAGE_HEADER_IS_RESPONSE       0
 #define PLDM_MESSAGE_HEADER_IS_DATAGRAM       1
 #define PLDM_MESSAGE_HEADER_INSTANCE_ID_MASK  0x1f