Fix #emit SYSREQ.C generating wrong native index for unused natives

This commit fixes a bug where the compiler generated incorrect native
index in assembly code for natives invoked via #emit sysreq.c and not
called elsewhere with the normal syntax.

See 4) here: http://forum.sa-mp.com/showthread.php?t=355877

--------- test code --------

native print(const s[]);
native printf(const format[], ...);

main() {
	new string[] = "hi there!";
	printf(string);
	#emit push.adr string
	#emit push.c 4
	#emit sysreq.c print
	#emit sysreq.c printf
	#emit stack 8
}

----- end of test code -----