LU-18685 deb: fix broken EXTRAFLAGS

LU-18685 deb: fix broken EXTRAFLAGS

A change to allow separate packages to be built has broken handling
of EXTRAFLAGS in debian/rules. It should always use:

 EXTRAFLAGS="${EXTRAFLAGS} ..."

Fixes: 1a2119c54e ("LU-17462 build: make some deb packages optional")
Test-Parameters: trivial
Signed-off-by: Ake Sandgren <ake.sandgren@hpc2n.umu.se>
Change-Id: I44aa60fea474ae046ba89a6502ba9b6c635c6918
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57922
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>