graphics/gegl: fix build with clang when using Altivec

Clang doesn't like including altivec.h manually:
/usr/lib/clang/8.0.1/include/altivec.h:69:19: error: unknown type name 'vector'
static inline vector bool char __ATTRS_o_ai

PR: 239757
Approved by: linimon (mentor), gnome (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D21404