RISC-V: lib: Fix build error for 64-bit

Fixes the following build error from tinyconfig:

riscv64-unknown-linux-gnu-ld: kernel/sched/fair.o: in function `.L8':
fair.c:(.text+0x70): undefined reference to `__lshrti3'
riscv64-unknown-linux-gnu-ld: kernel/time/clocksource.o: in function `.L0 ':
clocksource.c:(.text+0x334): undefined reference to `__lshrti3'

Fixes: 7f47c73 ("RISC-V: Build tishift only on 64-bit")
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/lib/Makefile

@@ -3,6 +3,6 @@ lib-y	+= memcpy.o
 lib-y	+= memset.o
 lib-y	+= uaccess.o
 
-lib-(CONFIG_64BIT) += tishift.o
+lib-$(CONFIG_64BIT) += tishift.o
 
 lib-$(CONFIG_32BIT) += udivdi3.o