testsuite: XFAIL gfortran.dg/initialization_25.f90 properly

The test was disabled/XFAIL'd informally in r0-100012-gcdc6637d7c78ec,
but r15-3890-g34bf6aa41ba539 didn't realize this, causing a FAIL.

Fix that by marking it as XFAIL per the original intent.

gcc/testsuite/ChangeLog:
	PR fortran/35779
	PR fortran/116858

	* gfortran.dg/initialization_25.f90: Mark as XFAIL.