Fix unused warnings in dummy definition

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/pkparse.c b/library/pkparse.c
index 9d87a71..ffa91d3 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -250,6 +250,9 @@
                                               size_t pub_len)
 {
 #if !defined(MBEDTLS_PK_PARSE_EC_COMPRESSED)
+    (void) pk;
+    (void) pub;
+    (void) pub_len;
     return MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE;
 #else /* MBEDTLS_PK_PARSE_EC_COMPRESSED */
     mbedtls_ecp_keypair ecp_key;