genksyms: Mark is_reserved_word static

The genksyms keyword gperf hash provides a function is_reserved_word.
genksyms #includes the resulting generated file keywords.c, so the
function gets used only in the same source file that defines it.  Mark
is_reserved_word static, and regenerate the corresponding generated
file.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>