Fix build breakage in gdb/xml-support.c

The buildbots are showing that the previous change to
xml_fetch_content_from_file causes __wur warnings/errors:

  ../../binutils-gdb/gdb/xml-support.c: In function gdb::unique_xmalloc_ptr<char> xml_fetch_content_from_file(const char*, void*):
  ../../binutils-gdb/gdb/xml-support.c:1028:43: error: ignoring return value of size_t fread(void*, size_t, size_t, FILE*), declared with attribute warn_unused_result [-Werror=unused-result]
     fread (text.get (), 1, len, file.get ());
					     ^

This commit fixes it.

gdb/ChangeLog:
2017-10-19  Pedro Alves  <palves@redhat.com>

	* xml-support.c (xml_fetch_content_from_file): Check fread's
	return.