Pick cortex-m-rt ^v0.6.15 to prevent multiple linking of cortex-m-rt

https://github.com/rust-embedded/cortex-m-rt/blob/master/CHANGELOG.md#backport-release-v0615---2021-07-12

Author: richardeoin

Cargo.toml

@@ -28,7 +28,7 @@ targets = ["thumbv7em-none-eabihf"]
 embedded-hal = "0.2.4"
 embedded-dma = "0.1.2"
 cortex-m = "^0.7.1"
-cortex-m-rt = "^0.6.12"
+cortex-m-rt = "^0.6.15"
 stm32h7 = "^0.13.0"
 void = { version = "1.0.2", default-features = false }
 cast = { version = "0.2.3", default-features = false }