examples: add return in spi_max7219.c

Some compilers are raising an error if no value is returned in main.
Add a return 0 in spi_max7219.c main to quiet them.

Signed-off-by: Cédric Bosdonnat <cbosdonnat@suse.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

examples/spi_max7219.c

@@ -85,4 +85,5 @@ main(int argc, char** argv)
     mraa_spi_stop(spi);
 
     //! [Interesting]
+    return 0;
 }