remove execinfo.h header

vlib/builtin/string.v

@@ -58,15 +58,10 @@ pub:
 	len   int
 }
 
-// For C strings only
-fn C.strlen(s byteptr) int
-
 pub fn vstrlen(s byteptr) int {
 	return C.strlen(*char(s))
 }	
 
-fn todo() { }
-
 // Converts a C string to a V string.
 // String data is reused, not copied.
 pub fn tos(s byteptr, len int) string {