Add server only guards for psk callback

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index eda6bc2..5a02182 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1390,10 +1390,12 @@
 #endif
 
 #if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
+#if defined(MBEDTLS_SSL_SRV_C)
     /** Callback to retrieve PSK key from identity                          */
     int (*MBEDTLS_PRIVATE(f_psk))(void *, mbedtls_ssl_context *, const unsigned char *, size_t);
     void *MBEDTLS_PRIVATE(p_psk);                    /*!< context for PSK callback           */
 #endif
+#endif
 
 #if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) && defined(MBEDTLS_SSL_SRV_C)
     /** Callback to create & write a cookie for ClientHello verification    */
@@ -3415,6 +3417,7 @@
                                    mbedtls_svc_key_id_t psk );
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
+#if defined(MBEDTLS_SSL_SRV_C)
 /**
  * \brief          Set the PSK callback (server-side only).
  *
@@ -3457,6 +3460,7 @@
                      int (*f_psk)(void *, mbedtls_ssl_context *, const unsigned char *,
                                   size_t),
                      void *p_psk );
+#endif /* MBEDTLS_SSL_SRV_C */
 #endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */
 
 #if defined(MBEDTLS_DHM_C) && defined(MBEDTLS_SSL_SRV_C)