Fix MicroBlaze __backtrace get_frame_size namespace (bug 21022).

Many linknamespace tests fail for MicroBlaze because __backtrace (as
brought in by libc_fatal.c) uses an inline function get_frame_size
which is not declared static.  This patch fixes it to be declared
static.

Tested (compilation tests) with build-many-glibcs.py.

	[BZ #21022]
	* sysdeps/microblaze/backtrace.c (get_frame_size): Make static.