Mark _dl_catch_error hidden

Since _dl_catch_error is only used internally in ld.so, it should be
declared in sysdeps/generic/ldsodefs.h, not include/dlfcn.h and it can
be made hidden.

	[BZ #19122]
	* include/dlfcn.h (_dl_catch_error): Moved to ...
	* sysdeps/generic/ldsodefs.h (_dl_catch_error): Add
	attribute_hidden.