Make backend features (native, intel-mkl, openblas) imply std

These backends expose HPC functionality via the hpc module which is
gated behind cfg(feature = "std"). Without this, --no-default-features
--features native fails to find hpc::ocr_simd and other hpc modules.

Also adds required-features = ["std"] to ocr_benchmark example as a
safeguard.

Co-authored-by: AdaWorldAPI <AdaWorldAPI@users.noreply.github.com>

Author: cursoragent

Cargo.toml

@@ -34,6 +34,9 @@ name = "ndarray"
 bench = false
 test = true
 
+[[example]]
+name = "ocr_benchmark"
+required-features = ["std"]
 
 [dependencies]
 num-integer = { workspace = true }