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/gen_loader.c

@@ -4,6 +4,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <asm/byteorder.h>
 #include <linux/filter.h>
 #include <sys/param.h>
 #include "btf.h"
@@ -13,8 +14,6 @@
 #include "hashmap.h"
 #include "bpf_gen_internal.h"
 #include "skel_internal.h"
-#include <asm/byteorder.h>
-#include "str_error.h"
 
 #define MAX_USED_MAPS	64
 #define MAX_USED_PROGS	32