Remove -Werror to avoid build failures

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/Makefile
===================================================================
--- a/Makefile
+++ b/Makefile
@@ -45,7 +45,7 @@
               > /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
 
 CFLAGS += -g
-CFLAGS += $(autodepend-flags) -Wall -Werror -Wno-frame-address
+CFLAGS += $(autodepend-flags) -Wall -Wno-frame-address
 frame-pointer-flag=-f$(if $(KEEP_FRAME_POINTER),no-,)omit-frame-pointer
 fomit_frame_pointer := $(call cc-option, $(frame-pointer-flag), "")
 fnostack_protector := $(call cc-option, -fno-stack-protector, "")