gfortran.dg/gomp/metadirective-3.f90: xfail on offload_nvptx

Currently, 'target' with a nested metadirective creating a 'teams' will
fail with a bogus error ("‘target’ construct with nested ‘teams’ construct
contains directives outside of the ‘teams’ construct").
That's tracked at PR118694 - and, hence, expected.

However, the testcase metadirective-3.f90 triggers this when compiling for
'target offload_nvptx' (otherwise, the code is optimized away). Use xfail to
silence the error as it is known and there is a tracking PR.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/metadirective-3.f90: Add xfail when
	compiling for offload_nvptx.