nvptx: Resolve 'flexible_array_member_type_p' conflict

    [...]/source-gcc/gcc/config/nvptx/nvptx.cc:2485:1: error: ‘bool flexible_array_member_type_p(const_tree)’ was declared ‘extern’ and later ‘static’ [-fpermissive]
     2485 | flexible_array_member_type_p (const_tree type)
          | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In file included from [...]/source-gcc/gcc/config/nvptx/nvptx.cc:30:
    [...]/source-gcc/gcc/tree.h:7022:13: note: previous declaration of ‘bool flexible_array_member_type_p(const_tree)’
     7022 | extern bool flexible_array_member_type_p (const_tree);
          |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

That's due to recent commit ed1d15e94f5ce28a9fdba6e3b255efef420c93f0
"c: harmonize checking for flex array member type".

	gcc/
	* config/nvptx/nvptx.cc (flexible_array_member_type_p): Rename to
	'nvptx_flexible_array_member_type_p'.