testing/gpscorrelate: upgrade to 2.1

https://github.com/dfandrich/gpscorrelate/releases/tag/2.1

Resolves a rebuild error with gcc 14.

gpx-read.c: In function 'ExtractTrackPoints':
gpx-read.c:133:33: error: implicit declaration of function 'abort' [-Wimplicit-function-declaration]
  133 |                                 abort();
      |                                 ^~~~~
gpx-read.c:39:1: note: include '<stdlib.h>' or provide a declaration of 'abort'
   38 | #include "latlong.h"
  +++ |+#include <stdlib.h>
   39 |
gpx-read.c:133:33: warning: incompatible implicit declaration of built-in function 'abort' [-Wbuiltin-declaration-mismatch]
  133 |                                 abort();
      |                                 ^~~~~
gpx-read.c:133:33: note: include '<stdlib.h>' or provide a declaration of 'abort'
gpx-read.c:137:42: error: implicit declaration of function 'atof' [-Wimplicit-function-declaration]
  137 |                         LastPoint->Lat = atof(Lat);
      |                                          ^~~~
gpx-read.c: In function 'FreeTrack':
gpx-read.c:310:17: error: implicit declaration of function 'free' [-Wimplicit-function-declaration]
  310 |                 free(CurrentFree);
      |                 ^~~~
gpx-read.c:310:17: note: include '<stdlib.h>' or provide a declaration of 'free'

cc: @Saijin-Naib