Move enum noside earlier in expression.h

For the expression rewrite series, I needed to move enum noside
earlier in expression.h.  Because this is a pure move, and because it
seems harmless and uncontroversial to move an enum definition earlier
in a file, I'm pushing it in early, to reduce the size of that series.

Tested by rebuilding.

gdb/ChangeLog
2020-12-21  Tom Tromey  <tom@tromey.com>

	* expression.h (enum noside): Move earlier.