build: Fix parallel build

Fix parallel build where parser.h won't have been generated when
we're trying to compile kword itself.

 YACC   tools/parser.c
 LEX    tools/lexer.c
conflicts: 3 shift/reduce
 CC     tools/kword.o
tools/kword.c:36:20: fatal error: parser.h: No such file or directory
compilation terminated.
1 file changed