Expose oacc_fn_attrib_level to be used in backends.

2018-12-19  Tom de Vries  <tdevries@suse.de>

	* omp-offload.c (oacc_fn_attrib_level): Remove static.
	* omp-offload.h (oacc_fn_attrib_level): Declare.

From-SVN: r267275