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: NRandy Dunlap <rdunlap@xenotime.net>
Acked-by: NRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NRichard 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.atSigned-off-by: NIngo Molnar <mingo@elte.hu>

arch/um/sys-i386/Makefile

@@ -8,7 +8,8 @@ obj-y = bug.o bugs.o checksum.o delay.o fault.o ksyms.o ldt.o ptrace.o \
 
 obj-$(CONFIG_BINFMT_ELF) += elfcore.o
 
-subarch-obj-y = lib/rwsem.o lib/string_32.o
+subarch-obj-y = lib/string_32.o
+subarch-obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += lib/rwsem.o
 subarch-obj-$(CONFIG_HIGHMEM) += mm/highmem_32.o
 subarch-obj-$(CONFIG_MODULES) += kernel/module.o