gdb: infrun: move stop_soon variable to inner scoped in handle_inferior_event

gdb: infrun: move stop_soon variable to inner scoped in handle_inferior_event

Moving it to an inner scope makes it clearer where it's used (only while
handling the TARGET_WAITKIND_LOADED event).

gdb/ChangeLog:

	* infrun.c (handle_inferior_event): Move stop_soon variable to
	inner scope.

Change-Id: Ic57685a21714cfbb38f1487ee96cea1d12b44652