Log Message:
x11/libdrm/buildlink3.mk: Pull in libpciaccess on x86.

This matches what the Makefile does.

Resolves the following annoying warnings (possibly harmful, unclear)
when building, e.g., graphics/wld:

Package pciaccess was not found in the pkg-config search path.
Perhaps you should add the directory containing `pciaccess.pc'
to the PKG_CONFIG_PATH environment variable
Package 'pciaccess', required by 'libdrm_intel', not found