Add missing include directive

src/matrix.c

@@ -9,6 +9,8 @@
 /// \file  matrix.c
 /// \brief Fixed-point 3D vector
 
+#include <string.h>
+
 #include "matrix.h"
 
 static matrix_t identitymatrix = {{

src/quaternion.c

@@ -9,6 +9,8 @@
 /// \file  quaternion.c
 /// \brief Fixed-point 3D vector
 
+#include <string.h>
+
 #include "quaternion.h"
 #include "vector3d.h"
 #include "matrix.h"

src/vector3d.c

@@ -9,6 +9,8 @@
 /// \file  vector3d.c
 /// \brief Fixed-point 3D vector
 
+#include <string.h>
+
 #include "vector3d.h"
 #include "r_main.h" // R_PointToDist2