apk: Fix host compilation with C89

This fixes the following build error:
```
../src/apk.c: In function 'parse_options':
../src/apk.c:584:4: error: a label can only be part of a statement and a declaration is not a statement
  584 |    char *arg = opt_parse_arg(&st);
      |    ^~~~
```

Upstream MR: https://gitlab.alpinelinux.org/alpine/apk-tools/-/merge_requests/376

Fixes: b91ebdabbb09 ("apk: bump to 3.0.1")
Signed-off-by: Hauke Mehrtens <[email protected]>

package/system/apk/patches/0020-apk-fix-compile-when-using-C89.patch

@@ -0,0 +1,56 @@
+From 54385e6dc02ada9ec827b6b79b9359951197fee5 Mon Sep 17 00:00:00 2001
+From: Paul Donald <[email protected]>
+Date: Tue, 9 Dec 2025 00:31:27 +0100
+Subject: [PATCH] apk: fix compile when using C89
+
+The older standard is more strict, and gives rise to errors:
+
+../src/apk.c: In function 'parse_options':
+../src/apk.c:584:4: error: a label can only be part of a statement and a declaration is not a statement
+  584 |    char *arg = opt_parse_arg(&st);
+      |    ^~~~
+
+So move the *arg declaration to function start.
+
+../src/app_mkpkg.c: In function 'mkpkg_setup_compat':
+../src/app_mkpkg.c:423:2: error: label at end of compound statement
+  423 |  default:
+      |  ^~~~~~~
+
+add break;
+
+Signed-off-by: Paul Donald <[email protected]>
+---
+ src/apk.c       | 3 ++-
+ src/app_mkpkg.c | 1 +
+ 2 files changed, 3 insertions(+), 1 deletion(-)
+
+--- a/src/apk.c
++++ b/src/apk.c
+@@ -556,6 +556,7 @@ static int parse_options(int argc, char
+ 	struct apk_opt_match m;
+ 	bool applet_arg_pending = false;
+ 	int r;
++	char *arg;
+ 
+ 	applet = applet_from_arg0(argv[0]);
+ 	if (!applet) {
+@@ -581,7 +582,7 @@ static int parse_options(int argc, char
+ 		case 0:
+ 			break;
+ 		case OPT_MATCH_NON_OPTION:
+-			char *arg = opt_parse_arg(&st);
++			arg = opt_parse_arg(&st);
+ 			if (applet_arg_pending && strcmp(arg, applet->name) == 0)
+ 				applet_arg_pending = false;
+ 			else if (arg[0] || !applet || !applet->remove_empty_arguments)
+--- a/src/app_mkpkg.c
++++ b/src/app_mkpkg.c
+@@ -421,6 +421,7 @@ static void mkpkg_setup_compat(struct mk
+ 	case 0: ctx->compat_rootnode = 1; // fallthrough
+ 	case 1: ctx->compat_dirnode = 1;  // fallthrough
+ 	default:
++		break;
+ 	}
+ }
+