sb/intel/ibexpeak/Makefile.inc: Remove path to non-existent folder

Found using 'Wmissing-include-dirs' command option.

Change-Id: I7c2217bbe677810d25c5d5d1062320773ee7e0c8
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70386
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

src/southbridge/intel/ibexpeak/Makefile.inc

@@ -33,6 +33,4 @@ romstage-y += early_cir.c
 romstage-y += early_usb.c
 romstage-y += setup_heci_uma.c
 
-CPPFLAGS_common += -I$(src)/southbridge/intel/ibexpeak/include
-
 endif