riscv: compat: vdso: Fix vdso_install target

riscv: compat: vdso: Fix vdso_install target

When CONFIG_COMPAT=y the vdso_install target fails:

$ make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- vdso_install
  INSTALL vdso.so
make[1]: *** No rule to make target 'vdso_install'.  Stop.
make: *** [arch/riscv/Makefile:112: vdso_install] Error 2

The problem is that arch/riscv/kernel/compat_vdso/Makefile doesn't
have a vdso_install target, but instead calls it compat_vdso_install.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20220625154207.80972-1-emil.renner.berthing@canonical.com
Fixes: 0715372a06ce ("riscv: compat: vdso: Add COMPAT_VDSO base code implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>