Fix missing declaration for symbol 'ngx_http_lua_ffi_exec_regex'

[vkill]

Hi there

When I run with official image and test it, the 500 happened. because OR require pcre-dev in runtime. please review it. thx.

docker run --name or_re_test_1 --rm -p 9001:81 -v $(pwd)/nginx.conf:/usr/local/openresty/nginx/conf/nginx.conf openresty/openresty:alpine
172.17.0.1 - - [07/Jul/2019:12:41:55 +0000] "GET /re HTTP/1.1" 500 183 "-" "curl/7.58.0"
2019/07/07 12:41:55 [error] 6#6: *1 lua entry thread aborted: runtime error: /usr/local/openresty/lualib/ngx/re.lua:47: missing declaration for symbol 'ngx_http_lua_ffi_exec_regex'
stack traceback:
coroutine 0:
	[C]: in function 'require'
	content_by_lua(nginx.conf:24):2: in main chunk, client: 172.17.0.1, server: , request: "GET /re HTTP/1.1", host: "127.0.0.1:9001"

The nginx.conf

worker_processes  1;

events {
    worker_connections 4;
}

http {
    server {
        listen 81;

        location /re {
            content_by_lua_block {
                local ngx_re = require "ngx.re"

                local res, err = ngx_re.split("a,b,c,d", ",")
                if err then
                    ngx.log(ngx.ERR, "failed: ", err)
                    return
                end

                for i = 1, #res do
                    ngx.say(res[i])
                end
            }
        }
    }
}

[vkill]

Fix #108

[neomantra]

Used the approach from this in a broader changeset. See fce29af.