Makefile.inc: Enable -Wimplicit-fallthrough

Change-Id: Ic81ed9eb2ed5255a221082326b81c375456a6499
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34300
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/Makefile.inc b/Makefile.inc
index 28f1363..2cad230 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -401,7 +401,7 @@
 
 CFLAGS_common += -pipe -g -nostdinc -std=gnu11
 CFLAGS_common += -nostdlib -Wall -Wundef -Wstrict-prototypes -Wmissing-prototypes
-CFLAGS_common += -Wwrite-strings -Wredundant-decls -Wno-trigraphs
+CFLAGS_common += -Wwrite-strings -Wredundant-decls -Wno-trigraphs -Wimplicit-fallthrough
 CFLAGS_common += -Wstrict-aliasing -Wshadow -Wdate-time -Wtype-limits
 CFLAGS_common += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
 CFLAGS_common += -ffunction-sections -fdata-sections -fno-pie