Disable default-features on rand dependency to avoid std version

Due to the single dependency tree, the attempted use of a std version
flips all depending crates to the std version as well which will not
compile on no_std systems.

Fixes #105

Signed-off-by: Daniel Egger <[email protected]>

Author: therealprof

cortex-m-rt/macros/Cargo.toml

@@ -14,12 +14,15 @@ proc-macro = true
 
 [dependencies]
 quote = "0.6.6"
-rand = "0.5.5"
 proc-macro2 = "0.4.15"
 
 [dependencies.syn]
 features = ["extra-traits", "full"]
 version = "0.14.8"
 
+[dependencies.rand]
+version = "0.5.5"
+default-features = false
+
 [dev-dependencies]
 cortex-m-rt = { path = "..", version = "0.6.0" }