rust: pci: fix a bug where build would fail when PCI is disabled

When building with allnoconfig, dmapool definitions are not picked up by
bindgen. Fix that.

Reported-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>

rust/bindings/bindings_helper.h

@@ -8,6 +8,7 @@
 
 #include <kunit/test.h>
 #include <linux/device.h>
+#include <linux/dmapool.h>
 #include <linux/errname.h>
 #include <linux/ethtool.h>
 #include <linux/interrupt.h>