libgomp/splay-tree.h: Fix splay_tree_prefix handling

libgomp/splay-tree.h: Fix splay_tree_prefix handling

When splay_tree_prefix is defined, the .h file
defines splay_* macros to add the prefix. However,
before those were only unset when additionally
splay_tree_c was defined.
Additionally, for consistency undefine splay_tree_c
also when no splay_tree_prefix is defined - there
is no interdependence either.

libgomp/ChangeLog:

	* splay-tree.h: Fix splay_* macro unsetting if
	splay_tree_prefix is defined.