Move security _pkey into X.509 condition (ARMmbed#99)

The private key member is only used when X.509 certificates are in use.
Move it into the condition X.509 condition.

This avoids compilation errors if mbed TLS is present with both X.509
and private key disabled.

source/coap_security_handler.c

@@ -51,8 +51,8 @@ struct coap_security_s {
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
     mbedtls_x509_crt            _cacert;
     mbedtls_x509_crt            _owncert;
-#endif
     mbedtls_pk_context          _pkey;
+#endif
 
     uint8_t                     _pw[64];
     uint8_t                     _pw_len;
@@ -109,8 +109,8 @@ static int coap_security_handler_init(coap_security_t *sec){
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
     mbedtls_x509_crt_init( &sec->_cacert );
     mbedtls_x509_crt_init( &sec->_owncert );
-#endif
     mbedtls_pk_init( &sec->_pkey );
+#endif
 
     memset(&sec->_cookie, 0, sizeof(simple_cookie_t));
     memset(&sec->_keyblk, 0, sizeof(key_block_t));
@@ -145,9 +145,8 @@ static void coap_security_handler_reset(coap_security_t *sec){
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
     mbedtls_x509_crt_free(&sec->_cacert);
     mbedtls_x509_crt_free(&sec->_owncert);
-#endif
-
     mbedtls_pk_free(&sec->_pkey);
+#endif
 
     mbedtls_entropy_free( &sec->_entropy );
     mbedtls_ctr_drbg_free( &sec->_ctr_drbg );