riscv: fix building external modules

When building external modules, vdso_prepare should not be run.  If the
kernel sources are read-only, it will fail.

Fixes: fde9c59 ("riscv: explicitly use symbol offsets for VDSO")
Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Makefile

@@ -107,11 +107,13 @@ PHONY += vdso_install
 vdso_install:
 	$(Q)$(MAKE) $(build)=arch/riscv/kernel/vdso $@
 
+ifeq ($(KBUILD_EXTMOD),)
 ifeq ($(CONFIG_MMU),y)
 prepare: vdso_prepare
 vdso_prepare: prepare0
 	$(Q)$(MAKE) $(build)=arch/riscv/kernel/vdso include/generated/vdso-offsets.h
 endif
+endif
 
 ifneq ($(CONFIG_XIP_KERNEL),y)
 ifeq ($(CONFIG_RISCV_M_MODE)$(CONFIG_SOC_CANAAN),yy)