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/elf.c

@@ -9,7 +9,6 @@
 #include <linux/kernel.h>
 
 #include "libbpf_internal.h"
-#include "str_error.h"
 
 /* A SHT_GNU_versym section holds 16-bit words. This bit is set if
  * the symbol is hidden and can only be seen when referenced using an