lib: int8_t set to signed char instead of char.

`char` signedness is not defined by C standard.  In some architectures
it's `unsigned` by default (RISC-V), so it's better to be explicit to
avoid problems.

* include/stdint.h (int8_t): Use `signed char` instead of `char`.

include/stdint.h

@@ -50,7 +50,7 @@
 #undef ptrdiff_t
 
 typedef unsigned char uint8_t;
-typedef char int8_t;
+typedef signed char int8_t;
 typedef unsigned short uint16_t;
 typedef short int16_t;
 typedef unsigned uint32_t;