uclibc-snapshot: enable option UCLIBC_HAS_OBSTACK

Prior to uClibc commit f143f920694c, uClibc used to always include the
gnu obstack extension, which is used and required by many tools, such as
binutils.

Because of the change mentioned above, obstack is now optional, and got
disabled per default, so enable it in the uClibc snapshot configuration file.

Signed-off-by: Chris Zankel <chris@zankel.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>