Correct relative include (#839)

This header file should pull in common.h from the local dir, instead of
relying upon include search order with include/pci_caps. This fixes the
build of SPDK.

Fixes: 23da2d88 ("Make libvfio-user headers self-contained (#834)")
Signed-off-by: John Levon <john.levon@nutanix.com>

include/pci_caps/msix.h

diff --git a/include/pci_caps/msix.h b/include/pci_caps/msix.h
index 0092c0c..c336e3d 100644
--- a/include/pci_caps/msix.h
+++ b/include/pci_caps/msix.h
@@ -37,7 +37,7 @@
 
 #include <linux/pci_regs.h>
 
-#include "pci_caps/common.h"
+#include "common.h"
 
 #ifdef __cplusplus
 extern "C" {