libbpf: Introduce errstr() for stringifying errno

Add function errstr(int err) that allows converting numeric error codes
into string representations.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241111212919.368971-2-mykyta.yatsenko5@gmail.com

src/str_error.h

@@ -6,4 +6,11 @@
 
 char *libbpf_strerror_r(int err, char *dst, int len);
 
+/**
+ * @brief **errstr()** returns string corresponding to numeric errno
+ * @param err negative numeric errno
+ * @return pointer to string representation of the errno, that is invalidated
+ * upon the next call.
+ */
+const char *errstr(int err);
 #endif /* __LIBBPF_STR_ERROR_H */