libmpd: fix build with strndup

libmpd: fix build with strndup

Retrieve a patch from upstream to include config.h otherwise build will
fail when trying to redefine strndup:
libmpd-internal.h:210:10: error: expected identifier or '(' before '__extension__'
 char *   strndup     (const char *s, size_t n);

Indeed, without an include on config.h, HAVE_STRNDUP won't be defined

Fixes:
 - http://autobuild.buildroot.org/results/a174818fa768b029d19b033139f9c5e0aaaed149

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