Makefile.mk: Enable string-compare command option

Change-Id: I7b05b6dd8f1de8689bfcc6825beb728111f6e54a
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81184
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/Makefile.mk b/Makefile.mk
index c901785..bfdb7e3 100644
--- a/Makefile.mk
+++ b/Makefile.mk
@@ -498,6 +498,7 @@
 CFLAGS_common += -Wdangling-else -Wmissing-include-dirs
 CFLAGS_common += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
 CFLAGS_common += -fstrict-aliasing -ffunction-sections -fdata-sections -fno-pie
+CFLAGS_common += -Wstring-compare
 ifeq ($(CONFIG_COMPILER_GCC),y)
 CFLAGS_common += -Wold-style-declaration
 # Don't add these GCC specific flags when running scan-build