libbpf: move libbpf_errstr() into libbpf_utils.c

Get rid of str_err.{c,h} by moving implementation of libbpf_errstr()
into libbpf_utils.c and declarations into libbpf_internal.h.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20251001171326.3883055-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>

src/libbpf_internal.h

@@ -172,6 +172,16 @@ do {				\
 #define pr_info(fmt, ...)	__pr(LIBBPF_INFO, fmt, ##__VA_ARGS__)
 #define pr_debug(fmt, ...)	__pr(LIBBPF_DEBUG, fmt, ##__VA_ARGS__)
 
+/**
+ * @brief **libbpf_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 *libbpf_errstr(int err);
+
+#define errstr(err) libbpf_errstr(err)
+
 #ifndef __has_builtin
 #define __has_builtin(x) 0
 #endif