libc: minimal: Add an implementation of iscntrl()

Implement the iscntrl() function, which returns whether a character is a
control one or not.

Ref: https://en.cppreference.com/w/c/string/byte/iscntrl

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

lib/libc/minimal/include/ctype.h

@@ -69,6 +69,11 @@ static inline int isalnum(int chr)
 	return (int)(isalpha(chr) || isdigit(chr));
 }
 
+static inline int iscntrl(int c)
+{
+	return (int)((((unsigned int)c) <= 31U) || (((unsigned int)c) == 127U));
+}
+
 #ifdef __cplusplus
 }
 #endif