treewide: Fix header includes to build with musl

Roughly inspired from a patch by Chris Kuhn: fix up includes so that
we can build against musl: glibc is more lenient as headers generally
include a larger amount of other headers.

Compared to the original patch, I only included what was needed
directly in C files, instead of adding blanket includes in local
header files. It's a bit more involved, but more consistent with the
current (not ideal) situation.

Reported-by: Chris Kuhn <kuhnchris+github@kuhnchris.eu>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.c

@@ -27,6 +27,8 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <netdb.h>
+#include <signal.h>
+#include <stdio.h>
 #include <string.h>
 #include <errno.h>
 #include <time.h>