Musl doesn't seem to include this by default so explicitly including it
should fix compilation on Musl.
Tested with Clang 16/17 and GCC 14.
Error:
src/backends/meta-fd-source.c:70:3: error: call to undeclared function 'close'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
close (fd_source->poll_fd.fd);
^
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3078
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3455>