feat: Update c_char type

(backport <#4195>)
(cherry picked from commit c66faeb)

src/unix/newlib/espidf/mod.rs

@@ -1,7 +1,7 @@
 use crate::prelude::*;
 
 pub type clock_t = c_ulong;
-pub type c_char = i8;
+pub type c_char = u8;
 pub type wchar_t = u32;
 
 pub type c_long = i32;