build: quiet LLVM non-literal string format warning

build: quiet LLVM non-literal string format warning

Compiling with LLVM generates a large number of warnings due
to the use of _() for wrapping strings for i18n:

    warning: format string is not a string literal
          (potentially insecure) [-Wformat-security]
    ./nls-enable.h:4:14: note: expanded from macro '_'
    #define _(a) (gettext (a))
                 ^~~~~~~~~~~~

These warnings are fixed by using "%s" as the format string,
and then _() is used as the string argument.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>