fs: move libc function prototypes to their correct locations ...

This moves the function prototypes in include/fs.h which also exist
in the libc to the locations they would have in libc.

With this it becomes easier to share code between barebox and userspace
since the usual libc include files will exist. Also users of the libc
functions no longer have to include the barebox internal fs.h header.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>