main: Fix memory leak in _mesa_make_extension_string()

I forgot to free the string returned by strdup().

Note: This is a candidate for the stable branches.
CC: Johannes Obermayr <johannesobermayr@gmx.de>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
1 file changed