Work around array out of bounds warning in mkdeps

Work around array out of bounds warning in mkdeps

This suppresses an array out of bounds warning in mkdeps.c as proposed
by Martin Sebor in the bugzilla.

array subscript 2 is outside array bounds of ‘const char [2]’

Since this warning does occur during bootstrap it currently breaks
werror builds on IBM Z.

The problem can be reproduced also on x86_64 by changing the inlining
threshold using: --param max-inline-insns-auto=80

Bootstrapped and regression tested on x86_64 and IBM Z.

libcpp/ChangeLog:

2020-01-16  Andreas Krebbel  <krebbel@linux.ibm.com>

	PR tree-optimization/92176
	* mkdeps.c (deps_add_default_target): Avoid calling apply_vpath to
	suppress an array out of bounds warning.