devel/opencl: don't include altivec.h

altivec.h shouldn't be included directly by users. It's managed with -maltivec / -mno-altivec compiler switches.

Including it causes compiler errors when using clang.

Already upstreamed.

PR: 247396
Approved by: ohartman@zedat.fu-berlin.de (maintainer timeout)
MFH: 2020Q3 (fixes build of some other ports)