Makefile: Enable -Wextra

Enable -Wextra. Exclude -Wno-override-init for now, because mx_getopt.h
relies on it [1].

[1]: #131

Makefile

@@ -122,7 +122,7 @@ CFLAGS_MYSQL += -DMX_MYSQL_FAIL_WAIT_DEFAULT=5
 
 CFLAGS += -g
 CFLAGS += -O3
-CFLAGS += -Wall
+CFLAGS += -Wall -Wextra -Wno-override-init
 CFLAGS += -DMXQ_VERSION=\"${MXQ_VERSION}\"
 CFLAGS += -DMXQ_VERSIONFULL=\"${MXQ_VERSIONFULL}\"
 CFLAGS += -DMXQ_VERSIONDATE=\"${MXQ_VERSIONDATE}\"