Add zalloc(size_t) allocator function

Same as calloc(1, len).
diff --git a/src/compositor.h b/src/compositor.h
index 7fbc4b3..7600ce3 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -37,6 +37,7 @@
 #include "version.h"
 #include "matrix.h"
 #include "config-parser.h"
+#include "zalloc.h"
 
 #ifndef MIN
 #define MIN(x,y) (((x) < (y)) ? (x) : (y))