Fix global_vars_f90_init test failure

Add a missing extern to ensure the test passes with -fno-common change.
Committed as obvious.

    testsuite/
	* gfortran.dg/global_vars_f90_init_driver.c: Add missing extern.

From-SVN: r278557

gcc/testsuite/ChangeLog

+2019-11-21  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	* gfortran.dg/global_vars_f90_init_driver.c: Add missing extern.
+
 2019-11-21  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/92596

gcc/testsuite/gfortran.dg/global_vars_f90_init_driver.c

 /* initialized by fortran */
-int i;
+extern int i;
 void test_globals(void);
 
 extern void abort(void);