powerpc: Update __NR_syscalls to account for SPU syscalls

A previous patch ended up not increasing __NR_syscalls to account
for the new SPU syscalls (probably my fault).
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/unistd.h

@@ -299,7 +299,7 @@
 #define __NR_spu_run		278
 #define __NR_spu_create		279
 
-#define __NR_syscalls		278
+#define __NR_syscalls		280
 
 #ifdef __KERNEL__
 #define __NR__exit __NR_exit