mesa: add stddef include in preparation for dropping p_compiler.h

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>

src/mesa/math/m_matrix.c

@@ -33,6 +33,7 @@
  * -# Transformation of a point p by a matrix M is: p' = M * p
  */
 
+#include <stddef.h>
 
 #include "c99_math.h"
 #include "main/errors.h"

src/mesa/math/m_vector.c

@@ -27,6 +27,7 @@
  */
 
 #include <stdio.h>
+#include <stddef.h>
 
 #include "main/glheader.h"
 #include "main/imports.h"