build: force building of the memtable plugin even if it wasn't specified

Makefile

@@ -230,6 +230,10 @@ dummy := $(shell (export ROCKSDB_ROOT="$(CURDIR)"; \
 # this file is generated by the previous line to set build flags and sources
 include make_config.mk
 
+ifeq ($(strip $(filter speedb,$(ROCKSDB_PLUGINS))),)
+ROCKSDB_PLUGINS += speedb
+endif
+
 ROCKSDB_PLUGIN_MKS = $(foreach plugin, $(ROCKSDB_PLUGINS), plugin/$(plugin)/*.mk)
 include $(ROCKSDB_PLUGIN_MKS)
 ROCKSDB_PLUGIN_PROTO =ROCKSDB_NAMESPACE::ObjectLibrary\&, const std::string\&