Fixes crash with #emit and unused functions

[VVWVV]

Hello!

The crash only happens when the parser skips the initialization of functions with SYSREQ. I.e. if we don't call the functions with #emit SYSREQ, the compiler crashs. BUT it doesn't work with public-functions.

Crash:

// We don't call the somefunc.
stock somefunc()
{
	#emit push.c 100
	#emit push.c 4

	#emit sysreq.c random

	#emit stack 8
	#emit retn
	return 0;
}

main()
{
	
}

Ok:

stock somefunc()
{
	#emit push.c 100
	#emit push.c 4

	#emit sysreq.c random

	#emit stack 8
	#emit retn
	return 0;
}

main()
{
	somefunc(); // We call it.
}

This PR adds a member variable called func_skipped in the symbol struct. This variable keeps track if the function was skipped. Also, in the #emit we check whether we need to mark the native-function as read or not.