Fix product name of GnuPG Desktop

* configure.ac (PRODUCT_NAME): Check for mym4_isgpd before checking for
mym4_isvsd.

For technical reason mym4_isvsd is set to "yes" also for gpd builds.
Therefore we need to check for mym4_isgpd before checking for mym4_isvsd
to determine the correct product name.