[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

gcc/testsuite/ChangeLog

+2019-06-24  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* gcc.dg/cpp/isysroot-1.c: Use <example.h> as the test header.
+	* gcc.dg/cpp/usr/include/stdio.h: Rename...
+	* gcc.dg/cpp/usr/include/example.h: ... to this.
+
 2019-06-24  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/90972

gcc/testsuite/gcc.dg/cpp/isysroot-1.c

 /* { dg-options "-isysroot ${srcdir}/gcc.dg/cpp" } */
 /* { dg-do compile  { target *-*-darwin* } } */
 
-#include <stdio.h>
+/* 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.  So use
+   a header name we don't expect to see. */
+#include <example.h>
 int main()
 {
-  /* Special stdio.h supplies function foo.  */
+  /* Special example.h supplies function foo.  */
   void (*x)(void) = foo;
   return 0;
 }