PCI: Add include guard to include/linux/pci_ids.h

Adding an include guard frees the preprocessor from reparsing over 2600
#defines in the cases where pci_ids.h is somehow included more than once.
This gives a tiny-but-measurable performance improvement when compiling
such files.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

include/linux/pci_ids.h

@@ -6,6 +6,8 @@
  *	Do not add new entries to this file unless the definitions
  *	are shared between multiple drivers.
  */
+#ifndef _LINUX_PCI_IDS_H
+#define _LINUX_PCI_IDS_H
 
 /* Device classes and subclasses */
 
@@ -2968,3 +2970,5 @@
 #define PCI_DEVICE_ID_XEN_PLATFORM	0x0001
 
 #define PCI_VENDOR_ID_OCZ		0x1b85
+
+#endif /* _LINUX_PCI_IDS_H */