gmacros: Ensure GUINT32/64_SWAP_LE_BE macros parenthesize arguments

Like all macros, we need to parenthesize arguments to ensure the order
of operations is correct.

See the mail thread starting at
<http://lists.fedoraproject.org/pipermail/devel/2013-March/180302.html>
"GCC produced wrong code in gvfs-1.14.2-3.fc18.x86_64" for how this
caused trouble with GVFS (which in turn caused trouble with
LibreOffice, where running "soffice sftp://.../.../test.odt" to access
an .odt file via GVFS failed to properly type-detect that file as a
Writer document and produced bogus error messages about the file being
broken).

https://bugzilla.gnome.org/show_bug.cgi?id=695925