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>

conf.c

@@ -23,8 +23,10 @@
 #include <limits.h>
 #include <grp.h>
 #include <pwd.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <stdint.h>
+#include <stdio.h>
 #include <stdbool.h>
 #include <unistd.h>
 #include <syslog.h>

isolation.c

@@ -65,6 +65,7 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 #include <time.h>
 #include <unistd.h>

netlink.c

@@ -18,6 +18,7 @@
 #include <errno.h>
 #include <sys/types.h>
 #include <limits.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <stdbool.h>
 #include <stdint.h>

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>

tap.c

@@ -14,6 +14,7 @@
  */
 
 #include <sched.h>
+#include <signal.h>
 #include <stdio.h>
 #include <errno.h>
 #include <limits.h>

tcp.c

@@ -267,6 +267,7 @@
 #include <sched.h>
 #include <fcntl.h>
 #include <stdio.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <errno.h>
 #include <limits.h>

tcp_splice.c

@@ -32,6 +32,7 @@
  */
 
 #include <sched.h>
+#include <signal.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>

udp.c

@@ -91,6 +91,7 @@
  */
 
 #include <sched.h>
+#include <signal.h>
 #include <stdio.h>
 #include <errno.h>
 #include <limits.h>

util.c

@@ -13,6 +13,7 @@
  */
 
 #include <sched.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <arpa/inet.h>