musl: add fallback parse_printf_format() implementation

musl does not provide parse_printf_format(). Let's introduce a fallback
method.

Co-authored-by: Yu Watanabe <watanabe.yu+github@gmail.com>

src/include/musl/printf.h

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/* Copyright 2014 Emil Renner Berthing <systemd@esmil.dk> */
+#pragma once
+
+#include <stddef.h>
+
+enum {              /* C type: */
+        PA_INT,     /* int */
+        PA_CHAR,    /* int, cast to char */
+        PA_WCHAR,   /* wide char */
+        PA_STRING,  /* const char *, a '\0'-terminated string */
+        PA_WSTRING, /* const wchar_t *, wide character string */
+        PA_POINTER, /* void * */
+        PA_FLOAT,   /* float */
+        PA_DOUBLE,  /* double */
+        PA_LAST,
+};
+
+/* Flag bits that can be set in a type returned by `parse_printf_format'.  */
+#define PA_FLAG_MASK        0xff00
+#define PA_FLAG_LONG_LONG   (1 << 8)
+#define PA_FLAG_LONG_DOUBLE PA_FLAG_LONG_LONG
+#define PA_FLAG_LONG        (1 << 9)
+#define PA_FLAG_SHORT       (1 << 10)
+#define PA_FLAG_PTR         (1 << 11)
+
+size_t parse_printf_format(const char *fmt, size_t n, int *types);