[Darwin, testsuite] Fix isystem-2.c.

[Darwin, testsuite] Fix isystem-2.c.

For the test to succeed there needs to be some header that is to be found in
the 'expected' place i.e. <sysroot>/usr/include/.  It's important that it is
not the name of a header for which fixincludes have been applied, since such
headers will be found in the gcc include-fixed dir and, in general, reference
additional headers.  The dummy sysroot will prevent the additional headers
from being found, resulting in a failed test.  The fix is to use a header name
that isn't expected to be present in a real sysroot.

2019-06-24  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.dg/cpp/isysroot-1.c (main): Use <example.h> as the test header.
	* gcc.dg/cpp/usr/include/stdio.h: Rename...
	* gcc.dg/cpp/usr/include/example.h: ... to this.

From-SVN: r272625