uaccess: Add a prototype of non-static __probe_user_read()

Declare a prototype of non-static __probe_user_read() as
same as __probe_kernel_read() at uaccess.h.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>