discover/grub2: distinguish parser.h from grub2/parser.h

discover/grub2: distinguish parser.h from grub2/parser.h

'make distcheck' will do a 'make all' srcdir == objdir, then later a
'make check' sith srcdir != objdir. Since gcc's cpp always assumes that
a source file's directory is first in the include paths, we may see
discover/parser.h included when we wanted the generated
discover/grub2/parser.h.

This change renames the grub2 lexer and parser files, to work-around
this behaviour, and fix 'make distcheck'.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>