package/mpd: needs gcc >= 8

package/mpd: needs gcc >= 8

Since bump to version 0.22.3 in commit b6576a458ca0 (package/mpd: bump
to version 0.22.3), mpd needs gcc >= 8, as documented in their manual
[0], to avoid the following build failure with gcc 7.3.1:

/tmp/instance-7/output-1/host/opt/ext-toolchain/aarch64-linux-gnu/include/c++/7.3.1/bits/stl_tree.h:2091:28: error: no matching function for call to 'std::_Rb_tree<std::__cxx11::basic_string<char>, std::pair<const std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::pair<const std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > > >::_M_get_insert_unique_pos(std::pair<std::basic_string_view<char>, std::basic_string_view<char> >::first_type&)'
  = _M_get_insert_unique_pos(_KeyOfValue()(__v));
    ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/4888d99404cc4273349ab036035c5ff7e086b83e

[0] https://mpd.readthedocs.io/en/stable/user.html#compiling-from-source)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: reword commit log to reference the manual]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>