Include stdio.h in dtostrf.c

This makes the declaration of sprintf available, so the function is not
implicitely declared, which triggers two compiler warnings.

This helps towards arduino#1792

Author: matthijskooijman

Diff for: cores/arduino/avr/dtostrf.c

@@ -18,6 +18,8 @@
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
+#include <stdio.h>
+
 char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
   char fmt[20];
   sprintf(fmt, "%%%d.%df", width, prec);