ggml-blas: explicitly link BLAS to fix undefined OpenBLAS symbols

[MetaPhaze]

ggml-blas uses cblas_* and openblas_set_num_threads but does not link
against BLAS, causing undefined references in downstream targets
(e.g. whisper-bench, test-vad).

This patch links BLAS::BLAS to the ggml-blas target, allowing CMake to
correctly propagate the dependency regardless of BLAS provider
(OpenBLAS, MKL, BLIS, Netlib).

Mention:

Gentoo

Ninja

Shared builds

Reproducible with OpenBLAS + GGML_USE_BLAS=ON

#if defined(GGML_USE_BLAS)
#  if __has_include(<cblas.h>)
#    include <cblas.h>
#  elif __has_include(<openblas/cblas.h>)
#    include <openblas/cblas.h>
#  else
#    error "cblas.h not found"
#  endif
#endif

[MetaPhaze]

include other markers for other blas targets