libc: Add getprogname function

defined here:
https://www.freebsd.org/cgi/man.cgi?query=getprogname&sektion=3

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

include/stdlib.h

@@ -265,6 +265,10 @@ FAR void  *bsearch(FAR const void *key, FAR const void *base, size_t nel,
                    size_t width, CODE int (*compar)(FAR const void *,
                    FAR const void *));
 
+/* Current program name manipulation */
+
+FAR const char *getprogname(void);
+
 #undef EXTERN
 #if defined(__cplusplus)
 }