urcu-defer: remove dependency on linux/futex.h

> cc -fPIC -Wall -I. -O2 -g -lpthread -c -o urcu-defer.o `echo urcu-defer.c urcu-defer.h | sed 's/[^ ]*\.h//g'`
> In file included from urcu-defer.c:31:
> /usr/include/linux/futex.h:96: error: expected ‘)’ before ‘*’ token
> /usr/include/linux/futex.h:100: error: expected ‘)’ before ‘*’ token

Seems broken on ppc. Just for two defines, it's not worth depending on it.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>