Fix warnings re: unused variables in pkparse and pkwrite

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/pkwrite.c b/library/pkwrite.c
index 0107f20..f229949 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -175,9 +175,12 @@
 int mbedtls_pk_write_pubkey(unsigned char **p, unsigned char *start,
                             const mbedtls_pk_context *key)
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
 
+    (void) p;
+    (void) start;
+    (void) key;
+
     PK_VALIDATE_RET(p != NULL);
     PK_VALIDATE_RET(*p != NULL);
     PK_VALIDATE_RET(start != NULL);
@@ -313,6 +316,10 @@
     unsigned char *c;
     size_t len = 0;
 
+    (void) ret;
+    (void) c;
+    (void) key;
+
     PK_VALIDATE_RET(key != NULL);
     if (size == 0) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;