Otherwise fails to build with missing size_t

diff --git a/src/zimg/colorspace/matrix3.cpp b/src/zimg/colorspace/matrix3.cpp
index 06e0e2f..bf849c0 100644
--- a/src/zimg/colorspace/matrix3.cpp
+++ b/src/zimg/colorspace/matrix3.cpp
@@ -1,4 +1,5 @@
 #include "matrix3.h"
+#include <sys/types.h>
 
 namespace zimg {
 namespace colorspace {