bpftool: Be more portable by using POSIX's basename()

musl libc had the basename() prototype in string.h, but this is a
glibc-ism, now they removed the _GNU_SOURCE bits in their devel distro,
Alpine Linux edge:

  https://git.musl-libc.org/cgit/musl/commit/?id=725e17ed6dff4d0cd22487bb64470881e86a92e7

So lets use the POSIX version, the whole rationale is spelled out at:

  https://gitlab.alpinelinux.org/alpine/aports/-/issues/15643Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <olsajiri@gmail.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/lkml/ZZhsPs00TI75RdAr@kernel.org
Link: https://lore.kernel.org/bpf/Zbe3NuOgaupvUcpF@kernel.org

tools/bpf/bpftool/gen.c

@@ -7,6 +7,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <libgen.h>
 #include <linux/err.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -56,9 +57,11 @@ static bool str_has_suffix(const char *str, const char *suffix)
 
 static void get_obj_name(char *name, const char *file)
 {
-	/* Using basename() GNU version which doesn't modify arg. */
-	strncpy(name, basename(file), MAX_OBJ_NAME_LEN - 1);
-	name[MAX_OBJ_NAME_LEN - 1] = '\0';
+	char file_copy[PATH_MAX];
+
+	/* Using basename() POSIX version to be more portable. */
+	strncpy(file_copy, file, PATH_MAX - 1)[PATH_MAX - 1] = '\0';
+	strncpy(name, basename(file_copy), MAX_OBJ_NAME_LEN - 1)[MAX_OBJ_NAME_LEN - 1] = '\0';
 	if (str_has_suffix(name, ".o"))
 		name[strlen(name) - 2] = '\0';
 	sanitize_identifier(name);