zlib: fix typo

Fix typo introduced by 1e32e0700a (remove dedicated target-headers option).
Noticed by Alexander Clouter <alex@digriz.org.uk>.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/zlib/zlib.mk

@@ -68,7 +68,7 @@ $(TARGET_DIR)/usr/lib/libz.a: $(STAGING_DIR)/usr/lib/libz.a
 	$(INSTALL) -D $(STAGING_DIR)/usr/lib/libz.a $(TARGET_DIR)/usr/lib/libz.a
 	touch -c $@
 
-zlib: $(ZLIB_TARGET) $(if $(BR2_HAVE_DEVFILES,$(TARGET_DIR)/usr/lib/libz.a)
+zlib: $(ZLIB_TARGET) $(if $(BR2_HAVE_DEVFILES),$(TARGET_DIR)/usr/lib/libz.a)
 
 zlib-source: $(DL_DIR)/$(ZLIB_SOURCE)