Make testing gdb with FORCE_SEPARATE_MI_TTY=1 actually work

Make testing gdb with FORCE_SEPARATE_MI_TTY=1 actually work

Runing the whole gdb testsuite with MI on a separate tty, with:

  make check RUNTESTFLAGS="FORCE_SEPARATE_MI_TTY=1"

Doesn't actually work because commit 51f77c3704a6 ("Add testing
infrastruture bits for running with MI on a separate UI") included a
last-minute rename typo, now fixed with this commit.

gdb/testsuite/ChangeLog:
2016-06-30  Pedro Alves  <palves@redhat.com>

	* lib/mi-support.exp (default_mi_gdb_start): Declare global
	FORCE_SEPARATE_MI_TTY, not SEPARATE_MI_TTY.