modify dgps_age type

change dgps_age type from int to struct minmea_float. 
example:  $--GGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh<CR><LF>

minmea.c

@@ -415,7 +415,7 @@ bool minmea_parse_gga(struct minmea_sentence_gga *frame, const char *sentence)
     int latitude_direction;
     int longitude_direction;
 
-    if (!minmea_scan(sentence, "tTfdfdiiffcfci_",
+    if (!minmea_scan(sentence, "tTfdfdiiffcfcf_",
             type,
             &frame->time,
             &frame->latitude, &latitude_direction,