dnf/mesa: Fix missing leading whitespace with ':append'

Mitigate occurrences where ':append' operator is used and leading
whitespace character is obviously missing, risking inadvertent
string concatenation.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>