Update zklua.c

[zhangskd]

Remove unexpected character '_', which leading to compile error below:

gcc -c pkg-config --cflags lua -fPIC -O2 -std=gnu99 zklua.c In file included from /usr/local/include/lua.h:13, from zklua.h:24, from zklua.c:29: /usr/lib/gcc/x86_64-redhat-linux/4.4.6/include/stddef.h:149: error: expected '=', ',', ';', 'asm' or 'attribute' before 'typedef'