lib: include endian.h for lcfs-internal.h

[nekopsykose]

required for htole16 and friends

---

fixes a build with clang16 against musl:

In file included from ../composefs/libcomposefs/lcfs-writer.c:21:
../composefs/libcomposefs/lcfs-internal.h:35:9: error: call to undeclared function 'htole16'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        return htole16(val);

this is because clang16 no longer permits using functions without a prototype first (implicit-decl)