PCI: cpcihp: Declare cpci_debug in header file

cpci_debug is declared as a global variable in cpci_hotplug_core.c and used
in cpci_hotplug_pci.c via an "extern". Add an extern declaration in the
header file.

Resolves the following sparse warning:

drivers/pci/hotplug/cpci_hotplug_core.c:47:5: warning: symbol 'cpci_debug' was not declared. Should it be static?

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Krzysztof Wilczyński <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>

drivers/pci/hotplug/cpci_hotplug.h

···
 int cpci_hp_start(void);
 int cpci_hp_stop(void);
 
+/* Global variables */
+extern int cpci_debug;
+
 /*
  * Internal function prototypes, these functions should not be used by
  * board/chassis drivers.

drivers/pci/hotplug/cpci_hotplug_pci.c

···
 
 #define MY_NAME	"cpci_hotplug"
 
-extern int cpci_debug;
-
 #define dbg(format, arg...)					\
 	do {							\
 		if (cpci_debug)					\