package/rust: disable ninja

package/rust: disable ninja

Ninja has recently be enabled as the default build system to build
llvm fork for rust compiler [1]. But we can still use Make if
"ninja = false" is provided in config.toml.

Ninja support can be enabled by a following patch.

[1] https://github.com/rust-lang/rust/commit/30b7dac745b1555cd96f41977f7d24435cbe7fa2

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1019386205

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>