Include "socket.h" for main_loop prototype. For C99 compatibility.

Also fixed via this in <https://github.com/ingwarsw/tinyfugue>:

commit 7b280f0f54dac62342c4f2a13a70b79bfb01a7b8
Author: Karol Lassak <ingwar@ingwar.eu.org>
Date:   Wed Dec 2 15:04:42 2020 +0000

    Add actions file (#31)
    
    Switch from travis to GH actions
    
    Also fixes for new C compilator and finding of lua and python libraries

diff --git a/src/tfio.c b/src/tfio.c
index 151583ebb17cd81e..bbbbd90f7de31134 100644
--- a/src/tfio.c
+++ b/src/tfio.c
@@ -45,6 +45,7 @@ static const char RCSid[] = "$Id: tfio.c,v 35004.114 2007/01/13 23:12:39 kkeys E
 #include "variable.h"	/* getvar() */
 #include "keyboard.h"	/* keyboard_pos */
 #include "expand.h"	/* current_command */
+#include "socket.h"	/* main_loop */
 #include "cmdlist.h"
 
 TFILE *loadfile = NULL; /* currently /load'ing file */