Revert "Remove gswe_init() call from GsweTimestamp initializers"

This reverts commit 078ff3370d.

It turns out that we actually need to call gswe_init() (or at least
g_type_init()) for GObjects to work.

src/gswe-timestamp.c

@@ -1341,6 +1341,8 @@ gswe_timestamp_new_from_gregorian_full(
 {
     GsweTimestamp *timestamp;
 
+    gswe_init();
+
     timestamp = GSWE_TIMESTAMP(g_object_new(GSWE_TYPE_TIMESTAMP,
                 "gregorian-year",            year,
                 "gregorian-month",           month,