[lldb] Explicitly mark PlatformFreeBSD's dependency on PlatformPOSIX

I accidentally broke the FreeBSD lldb-server build in 0c5cee77 because it
now depends on PlatformFreeBSD. PlatformFreeBSD depends on PlatformPOSIX
but this dependency was not explicitly tracked in CMake. As a result,
the FreeBSD lldb-server build broke.

Credit to John F. Carr <jfc@mit.edu> for pointing out the issue and
providing a fix.