Makefile: add -Wsign-compare to warning options

Now that all signedness comparison warnings in the source tree have been
fixed, let's enable the warning option, to avoid them creeping in again.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20210618172030.9684-6-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -21,7 +21,7 @@ CONFIG_LOCALVERSION =
 ASSUME_MASK ?= 0
 
 CPPFLAGS = -I libfdt -I . -DFDT_ASSUME_MASK=$(ASSUME_MASK)
-WARNINGS = -Wall -Wpointer-arith -Wcast-qual -Wnested-externs \
+WARNINGS = -Wall -Wpointer-arith -Wcast-qual -Wnested-externs -Wsign-compare \
 	-Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls -Wshadow
 CFLAGS = -g -Os $(SHAREDLIB_CFLAGS) -Werror $(WARNINGS) $(EXTRA_CFLAGS)