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.

(From OE-Core rev: 314041fd)

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0b6ca9beeff927bbb6158c71596ac475550559d8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>