fuse/guestunmount.c: Include <signal.h>

On macOS this fails with:

  guestunmount.c:170:5: error: call to undeclared function 'sigaction'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    170 |     sigaction (SIGINT, &sa, NULL);
        |     ^
  1 error generated.

Reported-by: Mohamed Akram
Related: https://github.com/libguestfs/libguestfs/pull/188

fuse/guestunmount.c

@@ -32,6 +32,7 @@
 #include <libintl.h>
 #include <poll.h>
 #include <limits.h>
+#include <signal.h>
 #include <sys/types.h>
 #include <sys/wait.h>