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>
(cherry picked from commit 36fef005b1)

src/mesa/main/extensions.c

@@ -747,6 +747,8 @@ get_extension_override( struct gl_context *ctx )
       }
    }
 
+   free(env);
+
    /* Remove trailing space. */
    len = strlen(extra_exts);
    if (extra_exts[len - 1] == ' ')