Move #pragma once outside #ifdef's.

In this particular case it does no harm,
but since in other cases it can lead to
problems I didn't want to add an exception
for this case to fix_headers.sh, that currently
chokes on this because it doesn't know better
than that it's a bad thing.

Note on how #pragma once works: when encountered
(aka the #ifdef that it is inside has to
be true), the compiler marks the whole
file as "seen" (this is implementation
defined, but most implementations store
the inode of the file). Subsequent #include's
of that file/inode are then completely skipped.
Hence it doesn't matter if the #pragma is at
the beginning, at the end or in the middle,
but it should be encountered every #include,
usually, and thus not be inside an #if... #endif
construct.