fs: d_path: include internal.h

make W=1 warns about a missing prototype that is defined but
not visible at point where simple_dname() is defined:

fs/d_path.c:317:7: error: no previous prototype for 'simple_dname' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <20230516195444.551461-1-arnd@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>