remove use of register keyword in C files

C compilers have been ignoring it for the last decade

Change-Id: I42918263121dd189bab9d27077798b779b9e8da1
Reviewed-on: https://gerrit.libreoffice.org/9873
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>