test: include headers for used funcs

These funcs use things like wait/write/read/strcmp but sometimes
don't include the right header for them.

* test/Makefile: Add -Wall to CFLAGS.
* test/clone.c: Include unistd.h.
* test/fork.c: Include sys/wait.h.
* test/sig.c: Include unistd.h.
* test/sigkill_rain.c: Include sys/wait.h.
* test/vfork.c: Include sys/wait.h.
* test/wait_must_be_interruptible.c: Include string.h

test/Makefile

@@ -1,3 +1,5 @@
+CFLAGS += -Wall
+
 all: \
     vfork fork sig skodic clone leaderkill childthread \
     sigkill_rain wait_must_be_interruptible threaded_execve

test/clone.c

@@ -4,6 +4,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <sched.h>
+#include <unistd.h>
 
 int child(void* arg)
 {

test/fork.c

@@ -1,5 +1,6 @@
 #include <stdlib.h>
 #include <unistd.h>
+#include <sys/wait.h>
 
 int main(int argc, char *argv[])
 {

test/sig.c

@@ -1,5 +1,6 @@
 #include <stdlib.h>
 #include <signal.h>
+#include <unistd.h>
 
 void interrupt()
 {

test/sigkill_rain.c

@@ -5,6 +5,7 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <stdio.h>
+#include <sys/wait.h>
 
 static const struct sockaddr sa;
 

test/vfork.c

@@ -1,5 +1,6 @@
 #include <stdlib.h>
 #include <unistd.h>
+#include <sys/wait.h>
 
 int main(int argc, char *argv[])
 {

test/wait_must_be_interruptible.c

@@ -3,6 +3,7 @@
 #include <signal.h>
 #include <sys/wait.h>
 #include <stdio.h>
+#include <string.h>
 
 /* Expected order is:
  * Child signals parent