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] 30b7dac745

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>

package/rust/rust.mk

@@ -65,6 +65,8 @@ define HOST_RUST_CONFIGURE_CMDS
 		echo 'channel = "stable"'; \
 		echo '[target.$(RUSTC_TARGET_NAME)]'; \
 		echo 'cc = "$(TARGET_CROSS)gcc"'; \
+		echo '[llvm]'; \
+		echo 'ninja = false'; \
 	) > $(@D)/config.toml
 endef