libmtp soname bump

[Linus Walleij <triad df lth se>]

I just built libmtp 0.2.0 in devel, bumping its soname from libmtp.so.5 to 
libmtp.so.6.

I'm still a bit clueless on how to pick up dependencies properly in devel, 
but apparently the builder was not populated by it immediately, because 
trying to rebuild gnomad2 to it failed (still picks up old soname).

Any advice is welcome...

Linus