Fix compiler warning with USE_COMMAND_TAGS 0

Jack

2017-02-22 db0bfe56689d8ca9bd2f716836db04d4cd252588

Fix compiler warning with USE_COMMAND_TAGS 0

The SCPI_CMD_LIST_END has a command tag.  If command tags are not being
used, this results in "excess elements in struct initializer" warnings
from AVR-GCC.

 libscpi/inc/scpi/types.h

@@ -113,7 +113,12 @@

    typedef struct _scpi_command_t scpi_command_t;

#if USE_COMMAND_TAGS
#define SCPI_CMD_LIST_END       {NULL, NULL, 0}
#else
    #define SCPI_CMD_LIST_END       {NULL, NULL}
#endif


    /* scpi interface */
    typedef struct _scpi_t scpi_t;