selftests/riscv: fix potential build failure during the "emit_tests" step

selftests/riscv: fix potential build failure during the "emit_tests" step

The riscv selftests (which were modeled after the arm64 selftests) are
improperly declaring the "emit_tests" target to depend upon the "all"
target. This approach, when combined with commit 9fc96c7c19df
("selftests: error out if kernel header files are not yet built"), has
caused build failures [1] on arm64, and is likely to cause similar
failures for riscv.

To fix this, simply remove the unnecessary "all" dependency from the
emit_tests target. The dependency is still effectively honored, because
again, invocation is via "install", which also depends upon "all".

An alternative approach would be to harden the emit_tests target so that
it can depend upon "all", but that's a lot more complicated and hard to
get right, and doesn't seem worth it, especially given that emit_tests
should probably not be overridden at all.

[1] https://lore.kernel.org/20230710-kselftest-fix-arm64-v1-1-48e872844f25@kernel.org

Fixes: 9fc96c7c19df ("selftests: error out if kernel header files are not yet built")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>