backends: Include unistd.h to fix compilation on musl

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>

src/backends/meta-fd-source.c

@@ -20,6 +20,8 @@
 
 #include "config.h"
 
+#include <unistd.h>
+
 #include "backends/meta-fd-source.h"
 
 typedef struct _MetaFdtSource