Make parser functions static where possible.

src/backend/parser/parse_agg.c

src/backend/parser/parse_clause.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_func.c

src/backend/parser/parse_node.c

src/backend/parser/parse_oper.c

src/backend/parser/parse_relation.c

src/backend/parser/parse_target.c

src/backend/utils/time/tqual.c

src/include/parser/gramparse.h

src/include/parser/parse_agg.h

src/include/parser/parse_clause.h

src/include/parser/parse_expr.h

src/include/parser/parse_func.h

src/include/parser/parse_node.h

src/include/parser/parse_oper.h

src/include/parser/parse_relation.h

src/include/parser/parse_target.h