libmpdclient: fix install

libmpdclient: fix install

By default, meson adds the build machine architecture for installation,
see: https://github.com/mesonbuild/meson/issues/2535
Use --libdir to override this default behavior

Fixes:
 - http://autobuild.buildroot.net/results/aec/aec9f3de545a832b66cd00c217fbaa99246e1715
 - http://autobuild.buildroot.net/results/07a/07adcba71106a2dca963e4071f4673791b41a603
 - http://autobuild.buildroot.net/results/a27/a272a8e521863374901a5e3c0fb9da0b8daa4076

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>