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.h

@@ -76,7 +76,7 @@ struct minmea_sentence_gga {
     struct minmea_float hdop;
     struct minmea_float altitude; char altitude_units;
     struct minmea_float height; char height_units;
-    int dgps_age;
+    struct minmea_float dgps_age;
 };
 
 enum minmea_gll_status {