build: add -Wno-error=unused-result to target cflags

Many glibc functions have __warn_unused_result__ in so many different
core functions, and failing the build for all of those simply does not
make any sense

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 47440

rules.mk

@@ -143,7 +143,7 @@ ifndef DUMP
     -include $(TOOLCHAIN_DIR)/info.mk
     export GCC_HONOUR_COPTS:=0
     TARGET_CROSS:=$(if $(TARGET_CROSS),$(TARGET_CROSS),$(OPTIMIZE_FOR_CPU)-openwrt-linux$(if $(TARGET_SUFFIX),-$(TARGET_SUFFIX))-)
-    TARGET_CFLAGS+= -fhonour-copts -Wno-error=unused-but-set-variable
+    TARGET_CFLAGS+= -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result
     TARGET_CPPFLAGS+= -I$(TOOLCHAIN_DIR)/usr/include
     ifeq ($(CONFIG_USE_MUSL),y)
       TARGET_CPPFLAGS+= -I$(TOOLCHAIN_DIR)/include/fortify