lib: compile uuid_guid_get_str if CONFIG_LIB_UUID=y

Currently uuid_guid_get_str() is only built if
CONFIG_PARTITION_TYPE_GUID=y.

To make it usable for other GUIDs compile it if CONFIG_LIB_UUID=y.
The linker will take care of removing it if it is unused.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>