vectorizer: Fix riscv build [PR120042]

vectorizer: Fix riscv build [PR120042]

r15-9859-ga6cfde60d8c added a call to dominated_by_p to tree-vectorizer.h
but dominance.h is not always included; and you get a build failure on riscv building
riscv-vector-costs.cc.

Let's add the include of dominance.h to tree-vectorizer.h

Pushed as obvious after builds for riscv and x86_64.

gcc/ChangeLog:

	PR target/120042
	* tree-vectorizer.h: Include dominance.h.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>