This extra _DEFAULT_SOURCE definition results in a double definition
which is a compile error.

This fixes the following compile error with glibc:
----------------------------------------------------------------------
ugps-2019-06-25-cd7eabcd/nmea.c:19: error: "_DEFAULT_SOURCE" redefined [-Werror]
 #define _DEFAULT_SOURCE

<command-line>: note: this is the location of the previous definition
cc1: all warnings being treated as errors

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>