Fix compilation of misc.c with gmtime_r replacement function for mingw

https://github.com/Hamlib/Hamlib/issues/784

src/misc.c

@@ -2346,6 +2346,27 @@ uint32_t CRC32_function(uint8_t *buf, uint32_t len)
     return crc ^ 0xFFFFFFFF;
 }
 
+#if defined(_WIN32)
+// gmtime_r can be defined by mingw
+#ifndef gmtime_r
+static struct tm *gmtime_r(const time_t *t, struct tm *r)
+{
+    // gmtime is threadsafe in windows because it uses TLS
+    struct tm *theTm = gmtime(t);
+
+    if (theTm)
+    {
+        *r = *theTm;
+        return r;
+    }
+    else
+    {
+        return 0;
+    }
+}
+#endif // gmtime_r
+#endif // _WIN32
+
 //! @cond Doxygen_Suppress
 char *date_strget(char *buf, int buflen)
 {