dsp: platform: Fix overlooked call to PLDM_MSGBUF_DEFINE_P()

In context, this should now be PLDM_MSGBUF_RW_DEFINE_P() for
const-correctness.

Fixes:

```
../src/dsp/platform.c:3550:2: error: call to undeclared function 'PLDM_MSGBUF_DEFINE_P'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
 3550 |         PLDM_MSGBUF_DEFINE_P(buf);
      |         ^
```

Fixes: 7a8d932bc3cd ("msgbuf: Define a separate msgbuf structure for encode/decode function")
Change-Id: I448395b6bc1f4914ca01f3edfd8c3cf7ce60a7d9
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/src/dsp/platform.c b/src/dsp/platform.c
index d1fdce6..2577f88 100644
--- a/src/dsp/platform.c
+++ b/src/dsp/platform.c
@@ -3547,7 +3547,7 @@
 	const struct pldm_platform_file_descriptor_pdr *pdr, void *data,
 	size_t *data_len)
 {
-	PLDM_MSGBUF_DEFINE_P(buf);
+	PLDM_MSGBUF_RW_DEFINE_P(buf);
 
 	if (!pdr || !pdr->file_name.ptr || !data ||
 	    (pdr->file_name.length <= 1)) {