kbuild: fix genksyms Makefile

When enabling GENERATE_PARSER the genksyms Makefile
failed to create _shipped version of generated files.

Modifying keywords.gperf failed to cause a rebuild
of genksyms.
Fixed by specifying keywowrds .c as explicit prerequisite
of the lexer.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>