Only switch on printf format warnings for error_exit() and friends when TOYBOX_DEBUG...

Only switch on printf format warnings for error_exit() and friends when TOYBOX_DEBUG enabled.

One again gcc manages to be incompetent about producing warnings. For example,
in bzcat we error_exit() with a message that's an error code fed into an
array of string constants, and apparently dereferencing an array of string
literals doesn't give you a string literal according to -Wformat-security.

Not breaking the code to humor the compiler here, the compiler is wrong.