Fix below warning by including "unistd.h" also

exec_signals_prepare.c:26:5: warning: implicit declaration of function 'execv' is invalid in C99
      [-Wimplicit-function-declaration]
    execv(argv[1], argv+1);
    ^
1 warning generated.

testsuite/tests/rts/exec_signals_prepare.c

@@ -2,6 +2,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <string.h>
+#include <unistd.h>
 
 // Invokes a process, making sure that the state of the signal
 // handlers has all been set back to the unix default.