drivers/xen-tpmfront: Fix compile issue with missing option.

Randy reports:

x86_64:

drivers/built-in.o: In function `xen_tpmfront_init':
xen-tpmfront.c:(.init.text+0x257c): undefined reference to `xenbus_register_frontend'

This is nicely fixed by selecting the XenBus frontend module.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/char/tpm/Kconfig

@@ -94,6 +94,7 @@ config TCG_ST33_I2C
 config TCG_XEN
 	tristate "XEN TPM Interface"
 	depends on TCG_TPM && XEN
+	select XEN_XENBUS_FRONTEND
 	---help---
 	  If you want to make TPM support available to a Xen user domain,
 	  say Yes and it will be accessible from within Linux. See