smstoold: only define USE_ICONV if it is enabled

Signed-off-by: John Crispin <john@phrozen.org>

utils/smstools3/Makefile

@@ -40,7 +40,7 @@ define Package/smstools3/description
 endef
 
 TARGET_CFLAGS += -D NUMBER_OF_MODEMS=1
-TARGET_CFLAGS += -D USE_ICONV
+TARGET_CFLAGS += $(if $(ICONV_FULL),-D USE_ICONV)
 TARGET_CFLAGS += -D DISABLE_INET_SOCKET
 TARGET_CFLAGS += -W -Wall
 TARGET_CFLAGS += -D_FILE_OFFSET_BITS=64