add format argument attributes to gettext function prototypes

add format argument attributes to gettext function prototypes

their absence completely breaks format string warnings in programs
with gettext message translations: -Wformat gives no results, and
-Wformat-nonliteral produces spurious warnings.

with gcc, the problem manifests only in standards-conforming profiles;
otherwise gcc sets these attributes by default for the gettext family.
with clang, the problem always manifests; clang has no such defaults.