util/mkerr.h: Restore header file rename
With '-internal', we commonly write the reason code macros to header
file renamed 'name.h' to 'nameerr.h'.  That renaming was removed by
mistake, this restores it.
Fixes
@@ -437,6 +437,7 @@ foreach my $lib ( keys %errorfile ) {
# Rewrite the header file
my $hfile = $hinc{$lib};
+ $hfile =~ s/.h$/err.h/ if $internal;
open( OUT, ">$hfile" ) || die "Can't write to $hfile, $!,";
print OUT <<"EOF";
/*
