Include unistd.h for getpid().

From Carsten Siegner.
diff --git a/lib/mkstemp.c b/lib/mkstemp.c
index 2ccd3a4..57d8848 100644
--- a/lib/mkstemp.c
+++ b/lib/mkstemp.c
@@ -43,6 +43,7 @@
 #endif
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #ifndef O_BINARY
 #define O_BINARY 0