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/arch/riscv/include/arch/pmp.h

@@ -9,7 +9,7 @@
  * this function needs to be implemented by a specific SoC.
  * return number of PMP entries for current hart
  */
-extern int pmp_entries_num(void);
+int pmp_entries_num(void);
 
 /* reset PMP setting */
 void reset_pmp(void);

src/arch/x86/include/arch/interrupt.h

@@ -7,7 +7,7 @@
 
 /* setup interrupt handlers for mainboard */
 #if CONFIG(PCI_OPTION_ROM_RUN_REALMODE)
-extern void mainboard_interrupt_handlers(int intXX, int (*intXX_func)(void));
+void mainboard_interrupt_handlers(int intXX, int (*intXX_func)(void));
 #elif CONFIG(PCI_OPTION_ROM_RUN_YABEL)
 #include <device/oprom/yabel/biosemu.h>
 #else