-Wpointer-sign: dwarf2read.c.

-Wpointer-sign: dwarf2read.c.

This fixes the remaining issues necessary to make the DWARF reader
-Wpointer-sign clean.  The 'filename' bit should be obvious.
'constant_pool' holds the contents of an obstack, which are 'char *'.
gdb_byte would work too, but it'd need more casts elsewhere, so I just
chose the minimal approach.  Any way would be fine with me.

gdb/
2013-04-19  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (dwarf2_get_dwz_file): Add cast to const char *.
	(read_index_from_section): Add cast to 'char *'.