timezone: declare 'tzdir' as 'const char'

fixes build warning
```
mateweather-timezone.c:74:11: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]

   74 |     tzdir = g_getenv ("TZDIR");
```

1 files changed, 2 insertions, 1 deletions

diff --git a/libmateweather/mateweather-timezone.c b/libmateweather/mateweather-timezone.c
index 182c4b0..3b4c790 100644
--- a/libmateweather/mateweather-timezone.c
+++ b/libmateweather/mateweather-timezone.c
@@ -61,7 +61,8 @@ static gboolean
 parse_tzdata (const char *tzname, time_t start, time_t end,
 	      int *offset, gboolean *has_dst, int *dst_offset)
 {
-    char *tzdir, *filename, *contents;
+    char *filename, *contents;
+    const char *tzdir;
     gsize length;
     int timecnt, transitions_size, ttinfo_map_size;
     int initial_transition = -1, second_transition = -1;