Require linker plugin for pr78768.c

The test-case has an xfail-ed line.  For linkers without plugin support, that
line happens to xpass.  Require linker with plugin support, such that the line
is no longer xpass-ing, but unsupported.

2017-05-01  Tom de Vries  <tom@codesourcery.com>

	* gcc.dg/pr78768.c: Require linker plugin.

From-SVN: r247443

gcc/testsuite/ChangeLog

+2017-05-01  Tom de Vries  <tom@codesourcery.com>
+
+	* gcc.dg/pr78768.c: Require linker plugin.
+
 2017-05-01  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	PR fortran/37131

gcc/testsuite/gcc.dg/pr78768.c

@@ -2,6 +2,7 @@
    by -flto
   { dg-do link }
   { dg-require-effective-target lto }
+  { dg-require-linker-plugin "" }
   { dg-options "-O2 -Walloca-larger-than=10 -Wformat -Wformat-overflow -flto" } */
 
 int main (void)