treewide: Remove 'extern' from functions declaration

"extern" is automatically implied with function declaration.

Change-Id: Ic40218acab5a009621b6882faacfcac800aaf0b9
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71890
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

src/include/device/device.h

@@ -207,7 +207,7 @@ bool is_enabled_pci(const struct device *pci);
 bool is_pci_dev_on_bus(const struct device *pci, unsigned int bus);
 
 /* Returns whether there is a hotplug port on the path to the given device. */
-extern bool dev_path_hotplug(const struct device *);
+bool dev_path_hotplug(const struct device *);
 
 /* Option ROM helper functions */
 void run_bios(struct device *dev, unsigned long addr);