um: Make rwsem.S depend on CONFIG_RWSEM_XCHGADD_ALGORITHM

rwsem.S can only be used with CONFIG_RWSEM_XCHGADD_ALGORITHM.
This unbreaks the UML build on i386.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: JBeulich@novell.com
Link: http://lkml.kernel.org/r/201107222055.35341.richard@nod.at
Signed-off-by: Ingo Molnar <mingo@elte.hu>