infra: fix autotools .la fixing when no .la files are present

Commit 6f638ea9 ("infra: /usr support:
STAGING_DIR can be outside BASE_DIR") changed a for-construct around 'sed
-i' in a 'find | xargs sed -i'. However, if the find returns no results,
sed complains with:
    '/bin/sed: no input files'

This commit adds the extra parameter -r / --no-run-if-empty to the xargs
call, effectively fixing this problem case.

Reported-by: Rohit Kumar <rkthebest@gmail.com>
Fix-suggested-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>