Use lstat unconditionally

Use lstat unconditionally

Since lstat gnulib module is imported, we can use it unconditionally.
lstat usage was introduced by this patch

https://sourceware.org/ml/gdb-patches/2012-01/msg00390.html

during the review, it was suggested to import gnulib lstat module, but
we didn't do that.

gdb:

2014-11-28  Yao Qi  <yao@codesourcery.com>

	* configure.ac (AC_CHECK_FUNCS): Remove lstat.
	* config.in, configure: Regenerate.
	* symfile.c (find_separate_debug_file_by_debuglink): Remove
	code checking HAVE_LSTAT is defined.