cmake : hardcode OpenBLAS library name when building in MSVC (Windows)

Most *nix like environments (including MSYS2 on Windows) have OpenBLAS
packages that allow coexistence of OpenBLAS builds with 32-bit and
64-bit interface (w/o and w/ OPENBLAS_USE64BITINT defined) and they
differ by not having or having "64" suffix in their library filenames.
That's not the case for OpenBLAS prebuilt libraries for Windows.

CMakeLists.txt

@@ -219,6 +219,11 @@ if (WHISPER_OPENBLAS)
         set(WHISPER_BLAS_LIB "openblas")
     endif ()
     set(BLA_PKGCONFIG_BLAS ${WHISPER_BLAS_LIB})
+    # OpenBLAS prebuilt libraries for Windows do not have "64" suffix in filename.
+    # (But .pc file has "64" suffix in filename for USE_64BITINT=1 Windows build.)
+    if (MSVC)
+        set(WHISPER_BLAS_LIB "openblas")
+    endif ()
 endif()
 
 if (WHISPER_BLAS)