swiotlb-xen: On x86-32 builts, select SWIOTLB instead of depending on it.

We used to depend on CONFIG_SWIOTLB, but that is disabled by default.
So when compiling we get this compile error:

arch/x86/xen/pci-swiotlb-xen.c: In function 'pci_xen_swiotlb_detect':
arch/x86/xen/pci-swiotlb-xen.c:48: error: lvalue required as left operand of assignment

Fix it by actually activating the SWIOTLB library.
Reported-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/Kconfig

@@ -74,6 +74,7 @@ config XEN_PLATFORM_PCI
 
 config SWIOTLB_XEN
 	def_bool y
-	depends on PCI && SWIOTLB
+	depends on PCI
+	select SWIOTLB
 
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