musl: introduce dummy function for gnu_get_libc_version()

As the header gnu/libc-version.h and gnu_get_libc_version() function
are glibc specific, and musl does not provide them.

src/include/musl/gnu/libc-version.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stddef.h>
+
+static inline const char* gnu_get_libc_version(void) {
+        return "";
+}