reduce version number to the smallest required version

Author: stlankes

src/libunwind/Cargo.toml

@@ -17,7 +17,7 @@ doc = false
 
 [dependencies]
 core = { path = "../libcore" }
-libc = { version = "0.2.65", features = ['rustc-dep-of-std'], default-features = false }
+libc = { version = "0.2.51", features = ['rustc-dep-of-std'], default-features = false }
 compiler_builtins = "0.1.0"
 cfg-if = "0.1.8"