Adds nfc_free() function to freed allocated buffers

Fixes issue 228 (Thanks to Alex Lian)

utils/nfc-scan-device.c

@@ -125,7 +125,7 @@ main(int argc, const char *argv[])
       if (verbose) {
         if (nfc_device_get_information_about(pnd, &strinfo) >= 0) {
           printf("%s", strinfo);
-          free(strinfo);
+          nfc_free(strinfo);
         }
       }
       nfc_close(pnd);

utils/nfc-utils.c

@@ -120,5 +120,5 @@ print_nfc_target(const nfc_target nt, bool verbose)
   char *s;
   str_nfc_target(&s, nt, verbose);
   printf("%s", s);
-  free(s);
+  nfc_free(s);
 }