Mark DriverRec exported in atimach64probe.h to match atimach64probe.c

Fixes build failure when compiling with Solaris Studio compiler using -xldscope=hidden:

"atimach64probe.c", line 308: redeclaration must have the same or more restrictive linker scoping: MACH64

[Upstreaming patch Solaris has been carrying locally since 2015.]