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.s string
	#emit push.c 4
	#emit sysreq.c print
	#emit sysreq.c printf
}

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

source/compiler/sc2.c

@@ -1224,6 +1224,13 @@ static int command(void)
             assert((sym->ident & iFUNCTN)!=0 || (sym->ident & iREFFUNC)!=0);
             stgwrite(sym->name);
           } else {
+            if (strcmp(name, "sysreq.c")==0) {
+              assert((sym->ident & iFUNCTN)!=0 && (sym->usage & uNATIVE)!=0);
+              if (sc_status==statWRITE && (sym->usage & uREAD)==0 && sym->addr>=0) {
+                /* reserve a SYSREQ id if called for the first time  */
+                sym->addr=ntv_funcid++;
+              } /* if */
+            }
             outval(sym->addr,FALSE);
           } /* if */
           /* mark symbol as "used", unknown whether for read or write */