Fix portability issue in script

If there was a reason for hardcoding the path, it should have been documented.
diff --git a/scripts/generate_errors.pl b/scripts/generate_errors.pl
index 0ee992d..b9a8e9c 100755
--- a/scripts/generate_errors.pl
+++ b/scripts/generate_errors.pl
@@ -46,7 +46,7 @@
 
 $/ = $line_separator;
 
-open(GREP, "/bin/grep \"define POLARSSL_ERR_\" $include_dir/* |") || die("Failure when calling grep: $!");
+open(GREP, "grep \"define POLARSSL_ERR_\" $include_dir/* |") || die("Failure when calling grep: $!");
 
 my $ll_old_define = "";
 my $hl_old_define = "";